Question 1

O is the centre of a circle and AB is a tangent to the circle touching it at point B. If OB = 3 cm and OA = 5 cm, where A is an external point lying on the tangent line, then what is the length of segment AB (in cm)?

Accepted Answer

Introduction / Context: This question tests your understanding of the geometry of a circle with a tangent line. A very important property in circle geometry is that the radius drawn to the point of tangency is perpendicular to the tangent. When a radius, a tangent segment, and the segment joining the centre to an external point are considered together, they naturally form a right triangle. This allows us to apply the Pythagoras theorem to find unknown lengths. Such problems are very common in aptitude tests and school level geometry because they combine fundamental circle properties with basic right triangle calculations. Given Data / Assumptions: • O is the centre of the circle. • AB is a tangent to the circle at point B. • OB is a radius of the circle, and OB = 3 cm. • OA is the segment from the centre to the external point A on the tangent, and OA = 5 cm. • The angle between OB and AB at B is 90 degrees because a radius is perpendicular to the tangent at the point of contact. • We are required to find the length of AB in centimetres. Concept / Approach: Since OB is perpendicular to AB at B, triangle OAB is a right angled triangle with right angle at B. In that triangle, OA is the hypotenuse, OB is one leg, and AB is the other leg. By the Pythagoras theorem for a right triangle, (hypotenuse)^2 = (leg1)^2 + (leg2)^2. Therefore, OA^2 = OB^2 + AB^2. Rearranging this equation allows us to solve for AB^2 and then take the square root to get AB. This is a direct and standard application of both circle properties and the Pythagoras theorem. Step-by-Step Solution: Step 1: Recognise that OB ⟂ AB because a radius is perpendicular to the tangent at the point of tangency. Step 2: Conclude that triangle OAB is a right triangle with right angle at B. Step 3: Apply the Pythagoras theorem: OA^2 = OB^2 + AB^2. Step 4: Substitute OA = 5 cm and OB = 3 cm into the equation: 5^2 = 3^2 + AB^2. Step 5: Compute the squares: 25 = 9 + AB^2. Step 6: Rearrange to find AB^2: AB^2 = 25 - 9 = 16, so AB = √16 = 4 cm. Verification / Alternative check: We can confirm the result by quickly checking Pythagoras in reverse: OB^2 + AB^2 = 3^2 + 4^2 = 9 + 16 = 25, which is exactly OA^2, and OA is 5 cm, so 25 is correct. This matches the standard 3, 4, 5 Pythagorean triple, which further supports that our triangle dimensions are consistent. There is no contradiction with the circle and tangent properties, so the answer is reliable. Why Other Options Are Wrong: Option √34 cm would correspond to AB^2 = 34, which would give OA^2 = 9 + 34 = 43, not 25, so it contradicts OA = 5 cm. Option 2 cm would give OA^2 = 3^2 + 2^2 = 9 + 4 = 13, which again does not match 25. Option 8 cm would give OA^2 = 9 + 64 = 73, far from 25. Hence all of these violate the Pythagoras relation with the given OA and OB values. Common Pitfalls: A typical mistake is to forget that the right angle is at B and to misapply Pythagoras with the wrong segment as the hypotenuse. Another error is to use subtraction incorrectly, such as computing 3^2 - 5^2 or reversing the hypotenuse and a leg. It is crucial to correctly identify OA as the longest side since it connects the centre to an external point and thus must be larger than the radius. Remembering the radius tangent perpendicular property and carefully writing the Pythagoras equation helps avoid these issues. Final Answer: The length of segment AB is 4 cm.

Question 2

In triangle ABC, medians AD and BE intersect at point G, the centroid. If D is the midpoint of BC and E is the midpoint of AC, then what is the ratio of the area of triangle BDG to the area of quadrilateral GDCE?

Accepted Answer

Introduction / Context: This problem explores how medians divide a triangle into smaller regions with specific area relationships. In triangle geometry, medians are segments joining a vertex to the midpoint of the opposite side, and their point of intersection is the centroid. The centroid has important properties, including dividing each median in a 2 : 1 ratio and partitioning the triangle into regions of equal or related areas. This question asks you to find the ratio between the area of one small triangle and the area of a larger quadrilateral formed by the medians inside triangle ABC. Given Data / Assumptions: • Triangle ABC is a non degenerate triangle. • D is the midpoint of BC, so AD is a median. • E is the midpoint of AC, so BE is a median. • The medians AD and BE intersect at point G, the centroid of triangle ABC. • We are asked to find the ratio of area(ΔBDG) to area(quadrilateral GDCE). Concept / Approach: A key concept is that the three medians of a triangle divide it into six smaller triangles of equal area. Although only two medians are explicitly mentioned, we can rely on this general fact to reason about the areas. Triangle BDG is one of those small triangles. Quadrilateral GDCE can be seen as the union of two of those small triangles, which leads naturally to a simple area ratio. Using symmetry and the equal area property rather than numeric coordinates keeps the solution conceptual and elegant, but a coordinate geometry approach would also confirm the same ratio. Step-by-Step Solution: Step 1: Recall that the three medians of a triangle intersect at the centroid and divide the triangle into 6 smaller triangles of equal area. Step 2: In triangle ABC, medians AD, BE and the third median from C (even if not drawn in the statement) partition the triangle into 6 congruent area regions. Step 3: Triangle BDG is one of those 6 small regions, so area(ΔBDG) is equal to 1/6 of the area of triangle ABC. Step 4: The quadrilateral GDCE can be viewed as the union of two of those small triangles, each having area equal to area(ΔBDG). Step 5: Therefore, quadrilateral GDCE has area equal to 2 * area(ΔBDG). Step 6: Hence the ratio area(ΔBDG) : area(GDCE) = 1 : 2. Verification / Alternative check: As an alternative, place triangle ABC on a coordinate plane with simple coordinates for A, B and C, find the midpoints D and E, determine the centroid G, and then compute the areas using the coordinate area formula. Such a calculation confirms that triangle BDG has area equal to one third of triangle BDC and that GDCE is composed of two equal small triangles each having the same area as BDG. Numerical verification matches the conceptual reasoning and again yields the ratio 1 : 2, so the result is stable regardless of the specific shape of triangle ABC. Why Other Options Are Wrong: Option 1 : 3 would imply that the quadrilateral has three times the area of triangle BDG, which would require it to contain three of the equal area small triangles, but it actually contains only two. Option 2 : 3 and 3 : 4 are ratios that do not match the equal six part structure produced by the medians, and a careful breakdown of the interior regions shows they are not consistent with the way areas are divided. Only 1 : 2 accurately reflects that GDCE is composed of exactly two regions whose areas are equal to that of BDG. Common Pitfalls: A common mistake is to assume without justification that the medians divide the triangle only into four regions or that these regions do not have equal area. Some learners also misidentify which smaller regions make up the quadrilateral GDCE and mistakenly count three or more small triangles instead of two. Remembering the theorem that three medians form 6 equal area triangles is crucial, and carefully visualising or sketching the triangle with all medians drawn helps avoid miscounting. Breaking down complex shapes into known smaller equal parts is a powerful geometry strategy. Final Answer: The required ratio of areas is 1 : 2.

Question 3

The diagonals of a rhombus measure 24 cm and 10 cm. What is the perimeter of this rhombus (in cm)?

Accepted Answer

Introduction / Context: This question is about a rhombus, a special type of parallelogram in which all sides are equal. A key geometric property of a rhombus is that its diagonals intersect at right angles and bisect each other. This property allows us to use right triangle relationships to find the length of a side when the diagonals are known. Once the side length is determined, calculating the perimeter simply involves multiplying the side length by four. Such problems test both your knowledge of properties of quadrilaterals and your ability to apply the Pythagoras theorem in a straightforward manner. Given Data / Assumptions: • The figure is a rhombus with all four sides equal in length. • Length of one diagonal = 24 cm. • Length of the other diagonal = 10 cm. • Diagonals of a rhombus intersect at right angles and bisect each other. • We need to find the perimeter of the rhombus in centimetres. Concept / Approach: When the diagonals of a rhombus intersect, they cut each other into two equal halves and meet at right angles. Hence, each half of the first diagonal is 24 / 2 = 12 cm, and each half of the second diagonal is 10 / 2 = 5 cm. Each side of the rhombus is the hypotenuse of a right triangle formed by these half diagonals. By applying the Pythagoras theorem in that right triangle, we can compute the side length. After that, the perimeter is simply 4 times the side length, since all sides of a rhombus are equal. Step-by-Step Solution: Step 1: Compute half of each diagonal, since diagonals bisect each other in a rhombus: half of 24 cm is 12 cm, half of 10 cm is 5 cm. Step 2: Consider the right triangle formed by these half diagonals and one side of the rhombus. The legs of this triangle are 12 cm and 5 cm. Step 3: Apply the Pythagoras theorem to find the side length s: s^2 = 12^2 + 5^2. Step 4: Compute the squares: 12^2 = 144 and 5^2 = 25, so s^2 = 144 + 25 = 169. Step 5: Take the square root: s = √169 = 13 cm. Step 6: The perimeter of the rhombus is 4 * s = 4 * 13 = 52 cm. Verification / Alternative check: The 5, 12, 13 triple is a well known Pythagorean triple, which gives further confidence that the calculations are correct. With side length 13 cm, all four sides are equal and the diagonals are consistent with the right triangle constructed. The perimeter of 52 cm is therefore logically consistent with all given dimensions and properties of a rhombus. No contradictions appear when the lengths and angle conditions are checked. Why Other Options Are Wrong: A perimeter of 56 cm would correspond to a side length of 14 cm, for which the associated right triangle would have hypotenuse 14 and legs 12 and 5, but 12^2 + 5^2 = 169, which is 13^2, not 14^2. Similarly, perimeters 68 cm and 72 cm imply even larger side lengths, which are not compatible with the given diagonal lengths when using the Pythagoras theorem. Hence, these options do not satisfy the geometric constraints of a rhombus with diagonals 24 cm and 10 cm. Common Pitfalls: Some students mistakenly take the full diagonals as the legs of the triangle instead of using half diagonals, leading to a wrong side length. Others forget that the diagonals of a rhombus are perpendicular, which is essential for applying the Pythagoras theorem correctly. It is important to remember the key properties: diagonals bisect each other at right angles and sides are all equal. Drawing a quick sketch and labelling half diagonals often helps avoid these conceptual mistakes. Final Answer: The perimeter of the rhombus is 52 cm.

Question 4

O is the centre of a circle. A point P lies outside the circle such that OP = 13 cm and the radius of the circle is 5 cm. A tangent from P touches the circle at T. What is the length of the tangent segment PT (in cm)?

Accepted Answer

Introduction / Context: This question is another classic application of the tangent from an external point to a circle. When you draw a tangent from a point outside a circle, the segment from the point of tangency to the centre is perpendicular to the tangent. This leads to a right triangle where the distance from the external point to the centre is the hypotenuse, the radius is one leg, and the tangent segment is the other leg. The Pythagoras theorem then gives a direct way to compute the length of the tangent segment. Such problems are widely used in aptitude tests to combine circle geometry with right triangle calculations. Given Data / Assumptions: • O is the centre of the circle. • P is an external point such that OP = 13 cm. • Radius of the circle, OT, is 5 cm. • PT is a tangent from P to the circle, touching it at T. • OT is perpendicular to PT at T, forming a right angle at T. • We need to find the length of segment PT in centimetres. Concept / Approach: In triangle OPT, OT is the radius and PT is a tangent, so angle ∠OTP is 90 degrees. Therefore, triangle OPT is right angled at T. In any right triangle, the Pythagoras theorem states that the square of the hypotenuse equals the sum of the squares of the other two sides. Here, OP is the hypotenuse, and OT and PT are the legs. Hence, OP^2 = OT^2 + PT^2. Rearranging gives PT^2 = OP^2 - OT^2, and taking the square root gives PT. This is a direct and efficient way to compute the length of the tangent segment. Step-by-Step Solution: Step 1: Recognise that OT ⟂ PT, so triangle OPT is a right triangle with right angle at T. Step 2: Use the Pythagoras theorem: OP^2 = OT^2 + PT^2. Step 3: Substitute OP = 13 cm and OT = 5 cm into the equation: 13^2 = 5^2 + PT^2. Step 4: Compute the squares: 13^2 = 169 and 5^2 = 25, so 169 = 25 + PT^2. Step 5: Rearrange to solve for PT^2: PT^2 = 169 - 25 = 144. Step 6: Take the square root: PT = √144 = 12 cm. Verification / Alternative check: We can quickly verify by checking that OT^2 + PT^2 equals OP^2: 5^2 + 12^2 = 25 + 144 = 169, which matches 13^2. This is the well known 5, 12, 13 Pythagorean triple, which indicates our calculation is correct. The tangent length must be less than the distance OP and greater than the radius, and 12 cm satisfies that expectation, lying between 5 cm and 13 cm, so the result is numerically reasonable as well. Why Other Options Are Wrong: Option √194 cm would give OT^2 + PT^2 = 25 + 194 = 219, which does not equal 169, so it is inconsistent with the Pythagoras theorem. Option 10 cm gives 25 + 100 = 125, again not equal to 169. Option 8 cm gives 25 + 64 = 89, still not 169. Therefore, all of these violate the basic right triangle relationship between OP, OT and PT in this configuration. Common Pitfalls: Students sometimes incorrectly treat OP as a leg rather than the hypotenuse or forget that the radius meets the tangent at a right angle. Another common error is to subtract in the wrong order, computing 5^2 - 13^2 instead of 13^2 - 5^2, which would yield a negative number under the square root. To avoid these mistakes, always identify the hypotenuse as the longest side opposite the right angle and write the Pythagoras formula with the hypotenuse on the left side: hypotenuse^2 = leg1^2 + leg2^2. Final Answer: The length of the tangent PT is 12 cm.

Question 5

In an equilateral triangle ABC, G is the centroid. If each side of the triangle is 9 cm (so AB = BC = CA = 9 cm), then what is the length of segment AG (in cm)?

Accepted Answer

Introduction / Context: This question deals with special properties of an equilateral triangle and its centroid. The centroid is the point where the three medians of a triangle intersect. In any triangle, the centroid divides each median in a 2 : 1 ratio, counting from the vertex to the midpoint of the base. In an equilateral triangle, medians, angle bisectors, perpendicular bisectors and altitudes all coincide, giving the triangle a high degree of symmetry. Using these facts, we can find the length from a vertex to the centroid when we know the side length. Given Data / Assumptions: • Triangle ABC is equilateral: AB = BC = CA = 9 cm. • G is the centroid of triangle ABC, where the three medians intersect. • In an equilateral triangle, each median is also an altitude and angle bisector. • We are required to find the distance AG in centimetres. Concept / Approach: First, we compute the length of a median (or altitude) of the equilateral triangle. For an equilateral triangle with side a, the length of the altitude is given by (√3 / 2) * a. This altitude is also a median from a vertex to the midpoint of the opposite side. The centroid divides this median in the ratio 2 : 1, with the longer segment adjoining the vertex. Hence, the length from the vertex to the centroid is 2 / 3 of the entire median length. By combining these facts, we can express AG in terms of the side length a and then substitute a = 9 cm. Step-by-Step Solution: Step 1: Let the side length of the equilateral triangle be a. Here, a = 9 cm. Step 2: Recall the formula for the altitude (which is also a median) of an equilateral triangle: altitude = (√3 / 2) * a. Step 3: Substitute a = 9 cm into the formula to get the median length: median = (√3 / 2) * 9 = (9√3) / 2 cm. Step 4: In any triangle, the centroid divides each median in the ratio 2 : 1, from the vertex to the midpoint of the opposite side. Step 5: Therefore, AG, which is the segment from vertex A to centroid G, is 2 / 3 of the full median: AG = (2 / 3) * median. Step 6: Substitute the median length: AG = (2 / 3) * (9√3 / 2) = (2 * 9√3) / (3 * 2) = 3√3 cm. Verification / Alternative check: We can also check the numeric value of 3√3 cm. Since √3 is approximately 1.732, 3√3 ≈ 3 * 1.732 ≈ 5.196 cm. This is slightly more than half of the side length, which makes sense because the centroid is located inside the triangle, closer to the base than to the midpoint of the altitude. Furthermore, if the entire median is (9√3) / 2 ≈ 7.794 cm, then 2 / 3 of this is indeed about 5.196 cm, consistent with our computation of AG. Why Other Options Are Wrong: If AG were 3 cm, that would be significantly smaller than 2 / 3 of the median and would contradict the 2 : 1 median division rule. The option 3√3 / 2 cm corresponds to one third of the median length, not two thirds, so it would place the point closer to the base than to the vertex, which is not the centroid. A length of 6 cm would exceed 2 / 3 of the median and even approach the full median, which is not consistent with the centroid location. Thus these options do not match the known geometric relationships. Common Pitfalls: A common mistake is to confuse the centroid with the circumcentre or incentre and use incorrect distances or ratios. Another error is to use the side length directly instead of first computing the median or altitude. Some learners also invert the ratio and take AG as 1 / 3 of the median rather than 2 / 3. To avoid such errors, remember that the centroid is the balancing point of the triangle and lies two thirds of the way from each vertex along each median, which is a fundamental and frequently tested property. Final Answer: The length of AG is 3√3 cm.

Question 6

In right angled triangle ABC, ∠B = 90° and AC is the hypotenuse. If m∠A = 30° and AC = 10 cm, then what is the length of side AB (in cm)?

Accepted Answer

Introduction / Context: This question uses basic trigonometry in a right angled triangle, specifically the properties of a 30°–60°–90° triangle. In such special triangles, the ratios of the sides are fixed: the side opposite 30° is half the hypotenuse, and the side opposite 60° is (√3 / 2) times the hypotenuse. Alternatively, you can use trigonometric functions such as sine and cosine to relate side lengths and angles. Problems like this are common in aptitude tests because they check whether you can quickly apply these well known ratios or simple trigonometric formulas. Given Data / Assumptions: • Triangle ABC is right angled at B, so ∠B = 90°. • AC is the hypotenuse of the triangle and AC = 10 cm. • Angle at A is ∠A = 30°. • Hence angle at C is ∠C = 60° by the angle sum property of a triangle. • We are required to find the length of side AB in centimetres. Concept / Approach: In a right triangle, the side opposite the right angle is the hypotenuse. Here, AC is the hypotenuse. Side AB is adjacent to angle A and side BC is opposite to angle A. Using cosine, cos(A) = adjacent / hypotenuse = AB / AC. For angle A = 30°, cos(30°) = √3 / 2. So AB = AC * cos(30°) = 10 * (√3 / 2). This gives the required length directly. Another way is to use the standard 30°–60°–90° ratio, where sides relative to the hypotenuse follow fixed proportions. Step-by-Step Solution: Step 1: In right triangle ABC, identify AC as the hypotenuse and AB as the side adjacent to angle A. Step 2: Write the cosine relation: cos(A) = AB / AC. Step 3: Substitute A = 30° and AC = 10 cm into the relation: cos(30°) = AB / 10. Step 4: Recall that cos(30°) = √3 / 2. Step 5: Substitute this value: √3 / 2 = AB / 10. Step 6: Solve for AB: AB = 10 * (√3 / 2) = 5√3 cm. Verification / Alternative check: We can also use the standard side ratio for a 30°–60°–90° triangle. The side opposite 30° (here BC) is half the hypotenuse, so BC = 10 / 2 = 5 cm. The side opposite 60° (here AB) is BC * √3 = 5 * √3, which agrees with the previous result. Numerically, 5√3 is about 5 * 1.732 ≈ 8.66 cm, which is less than the hypotenuse 10 cm and greater than the shorter leg 5 cm, so the lengths are in the correct order. Why Other Options Are Wrong: Option 5 cm is actually the length of the side opposite 30°, not the side adjacent, so it represents BC, not AB. Option 10√3 cm is larger than the hypotenuse and therefore cannot be a leg of the triangle. Option 10 cm is the hypotenuse itself, not one of the legs. Thus only 5√3 cm correctly matches the geometric and trigonometric relationships in the given triangle. Common Pitfalls: Students sometimes mix up which side is opposite and which is adjacent to the given angle, leading them to use sine instead of cosine or vice versa inappropriately. Another frequent error is to confuse the special triangle ratios and assume the side opposite 30° is (√3 / 2) times the hypotenuse, which is incorrect. Carefully drawing a quick diagram, labelling the angles and sides, and recalling that the side opposite 30° is the shortest leg can help ensure that you choose the correct formula and side ordering. Final Answer: The length of side AB is 5√3 cm.

Question 7

In a circle with centre O, chord AB subtends an angle θ with the tangent to the circle at point A. What is the measure of angle ∠ABO, where B is a point on the circle and OB is a radius?

Accepted Answer

Introduction / Context: This question examines your understanding of the relationships between tangents, radii, chords and angles in a circle. A key idea in circle geometry is that the radius drawn to the point of tangency is perpendicular to the tangent line. Additionally, angles formed by a chord and a tangent relate closely to angles inside the circle. Here, you are asked to connect the angle between a chord and a tangent at one point of the circle to an angle involving the radius at another point, using properties of isosceles triangles and right angles. Given Data / Assumptions: • There is a circle with centre O. • AB is a chord of the circle. • A tangent at point A makes an angle θ with the chord AB. • OA is a radius and is perpendicular to the tangent at A. • OB is another radius, joining O to point B on the circle. • We must find the angle ∠ABO, which is the angle at B between BA and BO. Concept / Approach: First, notice that the tangent at A is perpendicular to the radius OA, so the angle between OA and the tangent is 90°. The chord AB forms an angle θ with the tangent, so the angle between OA and AB can be obtained by subtracting θ from 90°. This angle at A between OA and AB is ∠OAB. Next, observe that OA and OB are both radii of the circle and hence are equal, so triangle AOB is isosceles. In an isosceles triangle, the base angles opposite the equal sides are equal, so ∠OAB equals ∠ABO. Therefore, once you find ∠OAB as 90° - θ, you can directly conclude the value of ∠ABO. Step-by-Step Solution: Step 1: Recognise that OA is perpendicular to the tangent at A, so the angle between OA and the tangent is 90°. Step 2: The chord AB makes an angle θ with the tangent at A, which means the angle between AB and the tangent is θ. Step 3: The angle between OA and AB, that is ∠OAB, is the difference between 90° and θ, so ∠OAB = 90° - θ. Step 4: Note that OA and OB are radii of the circle, so OA = OB, which makes triangle AOB an isosceles triangle. Step 5: In isosceles triangle AOB, the base angles at A and B are equal, so ∠OAB = ∠ABO. Step 6: Therefore, ∠ABO = 90° - θ. Verification / Alternative check: You can visualise triangle AOB: since the tangent at A is orthogonal to OA, and AB tilts away from the tangent by θ, it must be closer to OA by 90° - θ. Because triangle AOB is isosceles with OA = OB, the angle at B must mirror the angle at A. There is no need to know the actual numerical values of θ or any side lengths; the relationships between angles and equal sides determine the answer uniquely. This reasoning is consistent with general properties of isosceles triangles and tangents to circles. Why Other Options Are Wrong: Option θ would imply that ∠ABO equals the angle between the chord and the tangent directly, ignoring the 90° angle between the tangent and the radius, which is incorrect. Option 90° + θ would give an angle larger than 90° inside triangle AOB, which would conflict with the geometry of a triangle with a right angle already at A when considering OA and the tangent. Option 2 (180° - θ) is unrelated to the angle structure of triangle AOB and the given configuration, and does not follow from any standard theorem here. Only 90° - θ is consistent with all the circle and triangle properties involved. Common Pitfalls: A common mistake is to assume that the angle at B is simply θ or 90° + θ because students do not carefully track how many right angles are present and how the chord interacts with both the tangent and the radius. Another error is to forget that triangle AOB is isosceles, which is crucial for equating ∠OAB and ∠ABO. To avoid confusion, it is helpful to sketch the figure, mark the 90° angle between OA and the tangent, and then express all other angles in terms of θ, keeping in mind the equal side lengths OA and OB. Final Answer: The measure of angle ∠ABO is 90° - θ.

Question 8

Two adjacent sides of a parallelogram measure 21 cm and 20 cm. The diagonal joining the endpoints of these two sides is 29 cm. What is the area of the parallelogram (in square centimetres)?

Accepted Answer

Introduction / Context: This question combines properties of parallelograms with the Pythagoras theorem. A parallelogram has opposite sides parallel and equal, and its area is given by the product of the base and the corresponding height. However, when the height is not directly provided, you can sometimes use diagonals and side lengths to deduce special cases. Here, the lengths of two adjacent sides and the diagonal between them are given, which allows you to check whether the parallelogram is in fact a rectangle. Once you recognise this, finding the area becomes straightforward. Given Data / Assumptions: • The figure is a parallelogram with adjacent sides of lengths 21 cm and 20 cm. • The diagonal joining the endpoints of these two sides has length 29 cm. • The parallelogram lies in a Euclidean plane and has positive area. • We are asked to find the area of the parallelogram in square centimetres. Concept / Approach: Consider the triangle formed by the two adjacent sides and the included diagonal. Let these sides be a = 21 cm and b = 20 cm, and the diagonal be d = 29 cm. In a general parallelogram, the diagonal satisfies the law of cosines: d^2 = a^2 + b^2 - 2ab cos(θ), where θ is the angle between the sides. If this triangle turns out to be a right triangle, then cos(θ) = 0, which means θ = 90°. That would indicate that the parallelogram is actually a rectangle, and its area is simply a * b. So we test whether 21, 20, and 29 form a Pythagorean triple. Step-by-Step Solution: Step 1: Compute a^2 + b^2 for the given side lengths: 21^2 + 20^2. Step 2: Calculate: 21^2 = 441 and 20^2 = 400, so a^2 + b^2 = 441 + 400 = 841. Step 3: Compute d^2 for the diagonal: 29^2 = 841. Step 4: Since a^2 + b^2 = d^2, the triangle formed by sides 21 cm, 20 cm and diagonal 29 cm is a right triangle, and θ = 90°. Step 5: Therefore, the parallelogram is a rectangle with side lengths 21 cm and 20 cm. Step 6: The area of a rectangle is side1 * side2 = 21 * 20 = 420 square centimetres. Verification / Alternative check: The fact that 21^2 + 20^2 equals 29^2 exactly confirms the right angle between the sides. For a parallelogram with sides a and b and included angle θ, the area is a * b * sin(θ). Here, sin(90°) = 1, so the area reduces to a * b = 21 * 20 = 420, matching our calculation. There is no need for further adjustments or height calculations because the right angle already provides the maximum area for given side lengths. Why Other Options Are Wrong: Option 240 square centimetres would correspond to a smaller height than 20 cm for a base of 21 cm and does not reflect the right angle condition. Option 120 square centimetres is far too small and ignores the actual geometry of the sides and diagonal. Option 210 square centimetres is half of the correct result and may arise if someone mistakenly computes the area of the right triangle instead of the full parallelogram. Only 420 square centimetres correctly interprets the shape as a rectangle and uses the correct base and height. Common Pitfalls: Some students fail to check whether the parallelogram is a rectangle and instead try to guess the height or use incorrect formulas. Others may mistakenly think that the diagonal plays a direct role in the area formula. The key is to notice that the given side lengths and diagonal form a Pythagorean triple, signalling a right angle between the sides. Once that is recognised, the problem becomes very simple. Always verify whether a given triple satisfies a^2 + b^2 = c^2, as this often simplifies geometry questions dramatically. Final Answer: The area of the parallelogram is 420 square centimetres.

Question 9

In any triangle, the three medians are the line segments that join each vertex to the midpoint of the opposite side. The point at which all three medians meet is called the ______ of the triangle.

Accepted Answer

Introduction / Context: This is a conceptual question about special points associated with a triangle. Triangles have several important centres, such as the centroid, incentre, circumcentre and orthocentre, each defined in terms of a specific construction. Understanding these definitions is essential for geometry and aptitude exams, because many problems refer to these points or rely on their properties. Here, the definition focuses on medians, which are segments joining each vertex of the triangle to the midpoint of the opposite side. The point where all three medians intersect has a specific name and important properties related to balancing and area. Given Data / Assumptions: • A triangle has three medians, one from each vertex to the midpoint of the opposite side. • The question refers to the common point of intersection of all three medians. • We are asked to identify the correct name of this point from the given options: centroid, incentre, circumcentre and orthocentre. Concept / Approach: The key is to recall the definitions of the four classical triangle centres: • Centroid: point of intersection of the three medians. • Incentre: point of intersection of the internal angle bisectors, and centre of the inscribed circle touching all three sides. • Circumcentre: point of intersection of the perpendicular bisectors of the three sides, and centre of the circumscribed circle passing through all three vertices. • Orthocentre: point of intersection of the three altitudes of the triangle. Since the question explicitly mentions medians, we identify that the correct term is centroid. Step-by-Step Solution: Step 1: Recall that a median of a triangle is a line segment from a vertex to the midpoint of the opposite side. Step 2: Note that every triangle has exactly three medians, one from each vertex. Step 3: The three medians are always concurrent, meaning they intersect at a single point inside the triangle. Step 4: By definition, the point of intersection of the three medians is called the centroid of the triangle. Step 5: Therefore, the blank in the question should be filled with the word “centroid”. Verification / Alternative check: We can cross check by thinking about the other centres. The incentre is related to angle bisectors and equal distances from sides, not medians. The circumcentre is where perpendicular bisectors of sides meet, often outside the triangle for obtuse triangles. The orthocentre is where altitudes meet and may lie inside, on, or outside the triangle. Only the centroid is defined using medians, and it always lies inside the triangle and divides each median in the ratio 2 : 1 from the vertex to the base, which is consistent with the description given in the question. Why Other Options Are Wrong: “Incentre” is incorrect here because it is associated with internal angle bisectors and the incircle, not medians. “Circumcentre” is wrong because it relates to perpendicular bisectors and the circumcircle, not segments from vertices to midpoints. “Orthocentre” is the meeting point of altitudes, which are perpendicular from vertices to opposite sides, and does not involve midpoints in its definition. Therefore, none of these match the idea of intersection of medians, leaving “centroid” as the only correct term. Common Pitfalls: Students often confuse the four centres because their names sound similar and all refer to intersection points of certain lines in a triangle. A useful memory aid is to link the prefix to its construction: “cen” in centroid can be associated with “centre of mass,” matching the balancing point of medians; “in” in incentre suggests a circle inscribed inside the triangle; “circum” in circumcentre points to a circle drawn around the triangle; and “ortho” in orthocentre relates to orthogonal or perpendicular altitudes. Keeping these associations clear helps avoid mixing up the definitions. Final Answer: The point where the three medians of a triangle meet is called the centroid of the triangle.

Question 10

In right angled triangle ABC, ∠B = 90° and AC is the hypotenuse. D is the circumcentre of triangle ABC. If AB = 3 cm and BC = 4 cm, what is the length of BD (in cm)?

Accepted Answer

Introduction / Context: This question brings together the properties of a right angled triangle and its circumcentre. In any triangle, the circumcentre is the point where the perpendicular bisectors of the sides intersect, and it serves as the centre of the circle passing through all three vertices (the circumcircle). In a right angled triangle, there is a special and very useful fact: the circumcentre lies at the midpoint of the hypotenuse. This allows us to find distances from the circumcentre to the vertices easily using simple length relationships. Given Data / Assumptions: • Triangle ABC is right angled at B, so ∠B = 90°. • AC is the hypotenuse of triangle ABC. • D is the circumcentre of triangle ABC, that is, the centre of the circumcircle passing through A, B and C. • AB = 3 cm and BC = 4 cm. • We need to find the distance BD in centimetres. Concept / Approach: In a right angled triangle, the circumcentre lies at the midpoint of the hypotenuse. Therefore, D is the midpoint of AC. The radius of the circumcircle is thus half the hypotenuse length. Furthermore, the circumcentre is equidistant from all three vertices, so BD equals this radius and equals AD and CD. The first step is to compute the hypotenuse AC using the Pythagoras theorem, and then halve it to get the radius and therefore BD. Step-by-Step Solution: Step 1: Use the Pythagoras theorem in right triangle ABC to find AC: AC^2 = AB^2 + BC^2. Step 2: Substitute AB = 3 cm and BC = 4 cm: AC^2 = 3^2 + 4^2 = 9 + 16 = 25. Step 3: Take the square root to find AC: AC = √25 = 5 cm. Step 4: In a right angled triangle, the circumcentre lies at the midpoint of the hypotenuse, so D is the midpoint of AC. Step 5: Therefore, the radius of the circumcircle is half the hypotenuse: radius = AC / 2 = 5 / 2 = 2.5 cm. Step 6: Since the circumcentre is equidistant from all three vertices, BD equals the radius, so BD = 2.5 cm. Verification / Alternative check: We can verify by noting that AD = CD = BD = 2.5 cm, and D lies exactly midway between A and C. If we place triangle ABC on a coordinate plane with B at the origin, A on the x axis at (3, 0) and C on the y axis at (0, 4), then AC connects (3, 0) to (0, 4). The midpoint of AC is at ((3 + 0) / 2, (0 + 4) / 2) = (1.5, 2). The distance from B (0, 0) to D (1.5, 2) is √(1.5^2 + 2^2) = √(2.25 + 4) = √6.25 = 2.5 cm, confirming our result. Why Other Options Are Wrong: If BD were 3 cm or 4 cm, it would equal one of the legs and not half the hypotenuse, which contradicts the known circumcentre property for right triangles. A value of 5.5 cm would even exceed the hypotenuse length of 5 cm, which is impossible for a radius of the circumcircle that must be less than or equal to half the distance between the furthest pair of points. Thus these options are inconsistent with the geometry and the precise calculation of the hypotenuse. Common Pitfalls: Students sometimes confuse the circumcentre with the orthocentre, especially in right triangles, and may place the special point at the right angle vertex instead of at the midpoint of the hypotenuse. Another frequent error is to calculate the hypotenuse correctly but forget to divide by two to find the radius and distances from the circumcentre to the vertices. Keeping a clear picture that in a right triangle the circumcentre always lies at the midpoint of the hypotenuse avoids these mistakes and lets you answer such questions very quickly. Final Answer: The length of BD is 2.5 cms.

Question 11

The point inside a triangle that is equidistant from all three sides, and is the common point of intersection of the internal angle bisectors, is called the ______ of the triangle.

Accepted Answer

Introduction / Context: This conceptual question is about identifying one of the four classical centres of a triangle: the incentre, circumcentre, centroid and orthocentre. Each of these is defined in terms of a specific type of line drawn inside the triangle. The point being described here is equidistant from all three sides of the triangle and is formed by the intersection of the internal angle bisectors. Recognising which centre has this property is important in many geometry and aptitude problems where distances to sides or inscribed circles are involved. Given Data / Assumptions: • The point is equidistant from all three sides of the triangle. • It is the common point of intersection of all the internal angle bisectors of the triangle. • The problem asks for the correct name of this point among the standard triangle centres. Concept / Approach: We recall the definitions of the main triangle centres: • Incentre: intersection of the internal angle bisectors; centre of the inscribed circle (incircle) that touches all three sides; equidistant from all sides. • Circumcentre: intersection of the perpendicular bisectors of the sides; centre of the circumcircle passing through all three vertices; equidistant from all vertices, not sides. • Orthocentre: intersection of the three altitudes; not defined in terms of equal distances to sides or vertices in this way. • Centroid: intersection of the three medians; centre of mass but not equidistant from sides or vertices. Since the point described is equidistant from the sides and is formed by angle bisectors, it matches the definition of the incentre. Step-by-Step Solution: Step 1: Identify that the point is obtained by intersecting the internal angle bisectors of the triangle. Step 2: Recall that the unique point where the three internal angle bisectors meet is called the incentre of the triangle. Step 3: Note that the incentre is the centre of the incircle, which is a circle drawn inside the triangle and tangent to all three sides. Step 4: Because a circle is tangent to a line at exactly one point and has equal radius to each tangency point, the incentre is equidistant from all three sides. Step 5: Therefore, the blank should be filled with the term “Incentre”. Verification / Alternative check: Compare this with the other triangle centres. The circumcentre is equidistant from the vertices, not from the sides, so it cannot be the point described. The centroid is defined via medians, not angle bisectors, and does not maintain equal distance from either sides or vertices. The orthocentre is the point where altitudes meet and does not generally have any equal distance property like this. Only the incentre simultaneously satisfies being the intersection of internal angle bisectors and being equidistant from all sides because it is the centre of the inscribed circle that touches each side at exactly one point. Why Other Options Are Wrong: “Circumcenter” is incorrect here because it is tied to perpendicular bisectors of sides and equal distances to vertices, which is a different concept. “Orthocentre” is based on altitudes and does not guarantee equal distances to sides or vertices. “Centroid” is based on medians and represents the balance point of the triangle but does not maintain equal distances from the sides. Since the question clearly emphasises equal distance from sides and intersection of angle bisectors, only “Incentre” fits both criteria. Common Pitfalls: Many learners confuse the triangle centres because their names sound similar and they are often introduced together. A useful way to remember is to link words to their roles: “in” in incentre suggests a circle inscribed inside the triangle, “circum” in circumcentre suggests a circle around the triangle, “ortho” relates to perpendicular altitudes, and “cen” in centroid evokes the centre of mass via medians. Keeping these associations clear helps you quickly select the correct centre when a question describes specific geometric properties. Final Answer: The point equidistant from the sides of a triangle is called the Incentre of the triangle.

Question 12

A cone and a cylinder have equal heights. The ratio of the radii of their bases (cone : cylinder) is 2 : 1. What is the ratio of the volume of the cone to the volume of the cylinder?

Accepted Answer

Introduction / Context: This problem tests your understanding of the volumes of a right circular cone and a right circular cylinder when their heights are equal. Both solids are fundamental three dimensional shapes in mensuration. Many aptitude questions compare their volumes when certain dimensions are related through simple ratios. Knowing the volume formulas and being able to substitute ratios instead of actual numbers allows you to solve such problems quickly and without complex calculations. Given Data / Assumptions: • There is a right circular cone and a right circular cylinder. • The heights of the cone and the cylinder are equal; let this common height be h units. • The ratio of their radii (cone : cylinder) is 2 : 1. Let the radius of the cone be 2r and the radius of the cylinder be r. • We are asked to find the ratio of the volume of the cone to the volume of the cylinder. Concept / Approach: Recall the standard volume formulas: • Volume of a cylinder: V_cylinder = π * r^2 * h. • Volume of a cone: V_cone = (1 / 3) * π * R^2 * h, where R is the radius of the cone and h is the height. Since both solids have the same height and radii in ratio 2 : 1, we can express the radii using a common variable r and then compute the ratio of volumes symbolically. This approach keeps the calculations simple and independent of specific numerical heights or radii. Step-by-Step Solution: Step 1: Let the radius of the cylinder be r and its height be h. Step 2: Given that the radii are in the ratio 2 : 1 (cone : cylinder), the radius of the cone is 2r. Step 3: Write the volume of the cone: V_cone = (1 / 3) * π * (2r)^2 * h = (1 / 3) * π * 4r^2 * h = (4 / 3) * π * r^2 * h. Step 4: Write the volume of the cylinder: V_cylinder = π * r^2 * h. Step 5: Form the ratio V_cone : V_cylinder = (4 / 3) * π * r^2 * h : π * r^2 * h. Step 6: Cancel the common factors π, r^2 and h to get V_cone : V_cylinder = 4 / 3 : 1 = 4 : 3. Verification / Alternative check: You can choose simple numerical values to verify. For example, let r = 1 unit and h = 3 units. Then the cylinder's volume is π * 1^2 * 3 = 3π. The cone's radius is 2 units, so its volume is (1 / 3) * π * 2^2 * 3 = (1 / 3) * π * 4 * 3 = 4π. The ratio V_cone : V_cylinder is 4π : 3π, which simplifies to 4 : 3, matching our algebraic result. This confirms that the ratio does not depend on the specific numerical values of r and h as long as the given conditions are met. Why Other Options Are Wrong: A ratio of 3 : 4 would imply that the cone has smaller volume than the cylinder, which is not true when the cone's radius is larger and only the height is scaled in the same way. Ratios of 2 : 1 or 1 : 2 ignore the factor 1 / 3 present in the cone volume formula and the squaring of the radius, leading to incorrect comparisons. Only 4 : 3 correctly accounts for both the squared radius ratio and the 1 / 3 factor in the cone volume formula. Common Pitfalls: A common mistake is to forget the 1 / 3 factor in the cone volume formula and to write V_cone = π * R^2 * h instead of (1 / 3) * π * R^2 * h. Another error is to treat the radii linearly without squaring them, which would give a wrong ratio. Always remember that in volume formulas for solids of revolution, the radius usually appears squared (as in π * r^2), so ratio comparisons must square the scale factor for radii. Checking the formula carefully before substituting ratios helps avoid these errors. Final Answer: The ratio of the volume of the cone to the volume of the cylinder is 4:3.

Question 13

Which one of the following sets of three lengths can form the sides of a triangle under the triangle inequality rule?

Accepted Answer

Introduction / Context: This question tests your knowledge of the triangle inequality theorem, a fundamental rule in geometry that determines whether three given lengths can form a triangle. The theorem states that in any triangle, the sum of the lengths of any two sides must be strictly greater than the length of the remaining side. Aspirants for competitive exams must be very comfortable applying this rule quickly to check potential side combinations, since it appears frequently in aptitude tests and school level questions. Given Data / Assumptions: • Four different sets of three positive lengths are given as options. • We need to identify which set can represent the three sides of a triangle. • All lengths are in centimetres and are assumed to be exact side lengths, not approximate or ranges. Concept / Approach: The triangle inequality theorem has three conditions for each triple of side lengths a, b and c (with c being the largest): • a + b > c, • a + c > b, • b + c > a. If any one of these inequalities fails (that is, if the sum is less than or equal to the remaining side), then the three lengths cannot form a valid triangle. To solve this question efficiently, you can quickly sort or identify the largest side in each option, then check whether the sum of the other two sides is strictly greater than that largest side. Step-by-Step Solution: Step 1: Consider option (a): 9 cm, 6 cm, 2 cm. The largest side is 9 cm. Check 6 + 2 = 8, which is less than 9, so this set cannot form a triangle. Step 2: Consider option (b): 11 cm, 3 cm, 12 cm. The largest side is 12 cm. Check 11 + 3 = 14, which is greater than 12. For completeness, 11 + 12 > 3 and 3 + 12 > 11 are obviously true. So this set satisfies the triangle inequality and can form a triangle. Step 3: Consider option (c): 3 cm, 5 cm, 8 cm. The largest side is 8 cm. Check 3 + 5 = 8, which is equal to 8, not strictly greater. This represents a degenerate case, not a proper triangle, so the triangle inequality fails. Step 4: Consider option (d): 5 cm, 7 cm, 13 cm. The largest side is 13 cm. Check 5 + 7 = 12, which is less than 13, so this set cannot form a triangle. Step 5: Conclude that only option (b) satisfies all the triangle inequality conditions. Verification / Alternative check: For option (b) with sides 11 cm, 3 cm and 12 cm, we can quickly see that all conditions are met: 11 + 3 = 14 > 12, 11 + 12 = 23 > 3, and 3 + 12 = 15 > 11. This ensures that a non degenerate triangle exists. For the other options, at least one sum of two sides is less than or equal to the third side, preventing the formation of a proper triangle. Thus, the triangle inequality is applied correctly and unambiguously identifies the only valid choice. Why Other Options Are Wrong: In option (a), 6 + 2 = 8, which is less than 9, so a side would be too long to meet the others. In option (c), 3 + 5 = 8 exactly, which would make the three points collinear, leading to a degenerate triangle with zero area, which is not acceptable for most exam definitions of a triangle. In option (d), 5 + 7 = 12, which is less than 13, making one side longer than the sum of the other two, again impossible for a triangle. Therefore, all these violate the strict triangle inequality rule. Common Pitfalls: A common mistake is to treat the condition a + b ≥ c as sufficient and to allow equality, but equality leads only to a degenerate triangle. Exams usually expect strict inequalities. Another pitfall is to check only the sum of the two smallest sides without carefully identifying the largest side, or to forget to verify the other two inequalities even though they are usually satisfied once the largest side check passes. Developing the habit of quickly spotting the largest side and checking the sum of the remaining two against it will make triangle inequality problems much faster and more reliable to solve. Final Answer: The only valid combination that can form the sides of a triangle is 11 cm, 3 cm, 12 cm (option b).

Question 14

In a right angled triangle, the hypotenuse is 20 cm and the other two sides are in the ratio 4 : 3. What are the lengths (in cm) of these two legs of the triangle?

Accepted Answer

Introduction / Context: This problem is a standard application of the Pythagoras theorem in a right angled triangle when you are given the hypotenuse and the ratio of the remaining two sides. Such questions are very common in aptitude and competitive exams and help you practice working with integer Pythagorean triples. Given Data / Assumptions: The triangle is right angled. The hypotenuse has length 20 cm. The other two sides (legs) are in the ratio 4 : 3. Let the legs be 4k cm and 3k cm for some positive real number k. We use the Pythagoras theorem: (leg1)^2 + (leg2)^2 = (hypotenuse)^2. Concept / Approach: Whenever the ratio of the legs is given, a very efficient way is to express the unknown sides in terms of a common factor k. Then, apply the Pythagoras theorem and solve for k. Once k is known, you immediately get the exact lengths of the legs. Here, the ratio 4 : 3 hints at a scaled version of the classic 3 4 5 Pythagorean triple. Step-by-Step Solution: Step 1: Let the two legs be 4k cm and 3k cm. Step 2: Apply Pythagoras theorem: (4k)^2 + (3k)^2 = 20^2. Step 3: Compute the squares: 16k^2 + 9k^2 = 25k^2. Step 4: Set 25k^2 = 400 (since 20^2 = 400). Step 5: Divide both sides by 25: k^2 = 400 / 25 = 16. Step 6: Take the positive square root (lengths are positive): k = 4. Step 7: Therefore, the legs are 4k = 4 * 4 = 16 cm and 3k = 3 * 4 = 12 cm. Verification / Alternative check: Check the Pythagorean relation with the found lengths: 16^2 + 12^2 = 256 + 144 = 400, and the hypotenuse squared is 20^2 = 400. The equality holds exactly, so 16 cm and 12 cm are consistent. Also, the ratio 16 : 12 simplifies to 4 : 3, matching the given ratio of the legs. Why Other Options Are Wrong: 4 cm and 3 cm, 8 cm and 6 cm, and 12 cm and 9 cm all keep the 4 : 3 ratio but do not satisfy the Pythagoras theorem with hypotenuse 20 cm. For each of these, the sum of squares of the legs is not 400. The pair 10 cm and 8 cm does not even preserve the 4 : 3 ratio and also fails the Pythagorean condition for hypotenuse 20 cm. Common Pitfalls: A frequent mistake is to scale a known triple incorrectly or to forget to square the scaling factor when checking the Pythagoras relation. Another error is to choose numbers in the correct ratio but not check whether they fit the given hypotenuse. Always use (leg1)^2 + (leg2)^2 = hypotenuse^2 to confirm the final pair of sides. Final Answer: The two legs of the triangle are 16 cm and 12 cm.

Question 15

In triangle ABC, angle BAC = 90° and AD is drawn perpendicular to BC from A. If AD = 6 cm and BD = 4 cm, what is the length (in cm) of the hypotenuse BC?

Accepted Answer

Introduction / Context: This geometry question uses special properties of a right angled triangle when an altitude is drawn from the right angle to the hypotenuse. It is a classic configuration that allows you to relate the altitude and the segments of the hypotenuse using simple product relations, without needing trigonometry. Given Data / Assumptions: Triangle ABC has angle BAC = 90°, so A is the right angle. BC is the hypotenuse. AD is perpendicular to BC, so AD is the altitude from the right angle to the hypotenuse. AD = 6 cm. BD = 4 cm, where D lies on BC between B and C. We must find the full length BC. Concept / Approach: In a right angled triangle, when you drop an altitude from the right angle to the hypotenuse, several useful relations hold: AD^2 = BD * CD BD + CD = BC These allow you to find the unknown segment CD from the product relation, and then BC by adding BD and CD. This approach is often faster than coordinate methods or repeated use of Pythagoras theorem on multiple smaller triangles. Step-by-Step Solution: Step 1: Use the relation AD^2 = BD * CD for a right triangle with altitude to the hypotenuse. Step 2: Substitute AD = 6 cm and BD = 4 cm. Step 3: So 6^2 = 4 * CD, which gives 36 = 4 * CD. Step 4: Solve for CD: CD = 36 / 4 = 9 cm. Step 5: The hypotenuse BC is the sum of BD and CD, so BC = BD + CD = 4 + 9 = 13 cm. Verification / Alternative check: We can verify by reconstructing triangle ABC. One leg squared is AB^2 = BD * BC = 4 * 13 = 52. The other leg squared is AC^2 = CD * BC = 9 * 13 = 117. The sum AB^2 + AC^2 = 52 + 117 = 169, which is equal to BC^2 = 13^2 = 169. This confirms that BC = 13 cm is consistent with the Pythagoras theorem for the entire triangle ABC. Why Other Options Are Wrong: 10 cm, 12 cm, 15 cm and 11 cm do not satisfy the altitude relation AD^2 = BD * CD when BD = 4 cm and AD = 6 cm. If BC were one of these values, the derived CD and the subsequent checks with Pythagoras theorem would fail, so these options are not compatible with the given information. Common Pitfalls: Students often try to use Pythagoras theorem directly on smaller triangles without recalling the special product relations, which makes the problem longer. Another mistake is to assume BD and CD are equal, which is not generally true. Remembering the identity AD^2 = BD * CD in right triangles with an altitude to the hypotenuse helps solve such problems quickly and accurately. Final Answer: The length of BC is 13 cm.

Question 16

In triangle ABC, points D and E lie on sides AB and AC respectively, with DE parallel to BC. If AD : DB = 2 : 5 and the area of triangle ADE is 8 square cm, what is the ratio of the area of triangle ADE to the area of quadrilateral BDEC?

Accepted Answer

Introduction / Context: This question checks your understanding of similar triangles and how their areas scale with side lengths. When a line segment is drawn parallel to one side of a triangle, it creates a smaller, similar triangle, and area ratios can be derived directly from the ratio of corresponding sides. This is a common pattern in geometry and aptitude exams. Given Data / Assumptions: Triangle ABC has D on AB and E on AC. DE is parallel to BC. AD : DB = 2 : 5. The area of triangle ADE is 8 square cm. We need the ratio of area(ADE) : area(BDEC), where BDEC is the quadrilateral formed between DE and BC. Concept / Approach: When a line through a vertex is drawn parallel to the opposite side of a triangle, the smaller triangle is similar to the original triangle. If the ratio of a pair of corresponding sides is k, the ratio of their areas is k^2. Here, AD corresponds to AB in triangles ADE and ABC. Once we know the ratio of areas, we can find the area of the whole triangle ABC and then subtract the area of ADE to get the area of quadrilateral BDEC. Step-by-Step Solution: Step 1: From AD : DB = 2 : 5, we have AB = AD + DB = 2 + 5 = 7 parts. Step 2: So AD / AB = 2 / 7. Step 3: Since DE ∥ BC, triangles ADE and ABC are similar. Step 4: The ratio of their areas is the square of the ratio of corresponding sides: area(ADE) / area(ABC) = (AD / AB)^2 = (2 / 7)^2 = 4 / 49. Step 5: Let area(ABC) = X. Then 8 / X = 4 / 49, so X = 8 * 49 / 4 = 8 * 12.25 = 98 square cm. Step 6: The area of quadrilateral BDEC is area(ABC) − area(ADE) = 98 − 8 = 90 square cm. Step 7: So the desired ratio area(ADE) : area(BDEC) is 8 : 90, which simplifies by dividing both numbers by 2 to 4 : 45. Verification / Alternative check: We can verify the ratio directly from areas. area(ADE) / area(ABC) = 4 / 49 and area(BDEC) / area(ABC) = 45 / 49, because the remaining part of the triangle occupies the rest of the area. Then area(ADE) : area(BDEC) = 4 / 49 : 45 / 49 = 4 : 45, which agrees with our computation based on actual numbers. This confirms the correctness of the ratio. Why Other Options Are Wrong: 45 : 4 and 45 : 8 invert or distort the correct ratio, implying the quadrilateral is smaller than triangle ADE, which is impossible since it occupies most of the original triangle. 8 : 45 is close numerically to the simplified ratio but does not reduce to an integer ratio with area(ABC) consistent at 98 square cm. 1 : 4 would suggest that ADE is much smaller in area relative to BDEC than is actually the case in this configuration. Common Pitfalls: A common mistake is to use the given side ratio 2 : 5 directly for area ratios instead of squaring the scale factor 2 / 7. Another frequent error is forgetting that AB is the sum of AD and DB, leading to an incorrect scale factor. Always compute the full side ratio between corresponding sides first, then square it for area ratios when triangles are similar. Final Answer: The required ratio of areas is 4 : 45.

Question 17

In right angled triangle ABC, angle A = 90°. What is the value of cos²A + cos²B + cos²C?

Accepted Answer

Introduction / Context: This conceptual trigonometry question uses basic properties of angles in a triangle and the relationships between trigonometric functions of complementary angles. It asks for the sum cos²A + cos²B + cos²C in a right angled triangle, which is a neat identity that you can derive from simpler facts about right triangles. Given Data / Assumptions: Triangle ABC is right angled at A, so angle A = 90°. Therefore, angles B and C are acute and satisfy B + C = 90°. We must compute cos²A + cos²B + cos²C. Standard trigonometric identities for complementary angles apply: if B + C = 90°, then C = 90° − B. Concept / Approach: Use the fact that the cosine of 90° is zero, so cosA becomes very simple. Then, exploit that B and C are complementary. For complementary angles, cos and sin are co functions: cos(90° − θ) = sinθ. This leads to cosC = sinB and cos²C = sin²B. Combining cos²B and sin²B gives 1 due to the fundamental Pythagorean identity for trigonometric functions. Step-by-Step Solution: Step 1: Since A = 90°, cosA = cos90° = 0, so cos²A = 0² = 0. Step 2: We know B + C = 90°, hence C = 90° − B. Step 3: Using co function relation, cosC = cos(90° − B) = sinB. Step 4: Therefore cos²C = (sinB)² = sin²B. Step 5: Now consider cos²B + cos²C. Substitute cos²C = sin²B to get cos²B + sin²B. Step 6: By the basic identity sin²B + cos²B = 1 for any angle B, we obtain cos²B + cos²C = 1. Step 7: Finally, add cos²A: cos²A + cos²B + cos²C = 0 + 1 = 1. Verification / Alternative check: Pick a specific right triangle, for example with angles A = 90°, B = 30° and C = 60°. Then cos²A = cos²90° = 0, cos²B = cos²30° = (√3 / 2)² = 3 / 4 and cos²C = cos²60° = (1 / 2)² = 1 / 4. Adding gives 0 + 3 / 4 + 1 / 4 = 1. Any other valid right triangle angle pair will also satisfy the same identity because it rests on the general relation sin²θ + cos²θ = 1 and the complementary angle property. Why Other Options Are Wrong: 2 and 3 are too large and would require cos²B and cos²C to sum to more than 1, which is impossible since each cosine square is at most 1. 0 would imply cos²B and cos²C somehow cancel, which cannot happen with non negative squares. The value −1 is impossible because a sum of squares cannot be negative. Common Pitfalls: One common mistake is to confuse cos²A (cosine squared) with cos(2A) (cosine of double angle). Another error is to forget that B and C are complementary and miss the link cosC = sinB. Always check carefully whether the question uses a power (cos²) or a multiple angle (cos 2θ), and use the identity sin²θ + cos²θ = 1 whenever complementary angles are involved. Final Answer: The value of cos²A + cos²B + cos²C is 1.

Question 18

What are the coordinates of the reflection of the point (−3, 1) in the vertical line x = −2 in the coordinate plane?

Accepted Answer

Introduction / Context: This coordinate geometry problem asks you to reflect a point across a vertical line. Reflection questions are common in analytic geometry and help you understand symmetry with respect to lines like x = a or y = b in the Cartesian plane. Given Data / Assumptions: Original point P has coordinates (−3, 1). The line of reflection is the vertical line x = −2. We are asked to find the coordinates of the reflected point P'. Reflection across a vertical line changes only the x coordinate; the y coordinate remains the same. Concept / Approach: For reflection in a vertical line x = a, the x coordinate of a point and its reflection are equidistant from a but on opposite sides. If the original point has x coordinate x1, then the reflected point has x coordinate x2 such that: a − x1 = x2 − a or equivalently: x2 = 2a − x1 The y coordinate is unchanged because the line of reflection is vertical and symmetry is only left right across this line. Step-by-Step Solution: Step 1: Identify the line of reflection x = −2, so a = −2. Step 2: The original point P is (x1, y1) = (−3, 1). Step 3: Use the reflection formula for the x coordinate: x2 = 2a − x1. Step 4: Substitute a = −2 and x1 = −3: x2 = 2 * (−2) − (−3) = −4 + 3 = −1. Step 5: The y coordinate stays the same, so y2 = y1 = 1. Step 6: Therefore, the reflected point P' has coordinates (−1, 1). Verification / Alternative check: We can check distances to the line x = −2. For the original point P(−3, 1), the horizontal distance to the line is |−3 − (−2)| = |−1| = 1 unit to the left. For the reflected point (−1, 1), the horizontal distance is |−1 − (−2)| = 1 unit to the right. Both points have the same y coordinate and are symmetric with respect to x = −2, confirming that (−1, 1) is correct. Why Other Options Are Wrong: (1, 1) reflects the point across x = −1, not x = −2, so it is too far to the right. (−3, −5) and (−3, 5) keep the x coordinate unchanged and instead move the point vertically, which corresponds to reflection across a horizontal line, not a vertical one. (−5, 1) moves the point further left by 2 units rather than mirroring it across x = −2, so the distances from the line are not equal on both sides. Common Pitfalls: Students sometimes reflect in the wrong direction (up down instead of left right) or forget that reflection across a vertical line only changes the x coordinate. Another common mistake is to add the distance instead of using the formula x2 = 2a − x1, leading to asymmetric points. Drawing a quick sketch of the line and the point often helps visualize the correct reflection. Final Answer: The reflection of (−3, 1) in the line x = −2 is (−1, 1).

Question 19

The coordinates of the centroid of triangle ABC are (2, 2). If the coordinates of vertices A and B are (7, −1) and (1, 2) respectively, what are the coordinates of vertex C?

Accepted Answer

Introduction / Context: This question uses the centroid formula from coordinate geometry. The centroid of a triangle is the average of the coordinates of its three vertices. If the centroid and two vertices are known, you can reverse this relationship to find the third vertex. This is a useful technique in both pure math problems and practical geometry applications. Given Data / Assumptions: Triangle ABC has centroid G with coordinates (2, 2). Vertex A has coordinates (7, −1). Vertex B has coordinates (1, 2). Vertex C has unknown coordinates (x, y). The centroid of a triangle with vertices (x1, y1), (x2, y2) and (x3, y3) is ((x1 + x2 + x3) / 3, (y1 + y2 + y3) / 3). Concept / Approach: We use the centroid formula in reverse. Since we know the final centroid coordinates and two of the three vertices, we set up two equations: one for the x coordinate and one for the y coordinate. Solving these linear equations gives the missing coordinates of vertex C. This is a direct algebraic manipulation of the centroid definition. Step-by-Step Solution: Step 1: Let C have coordinates (x, y). Step 2: Apply the centroid formula for the x coordinate: (xA + xB + xC) / 3 = 2. Step 3: Substitute xA = 7, xB = 1 and xC = x: (7 + 1 + x) / 3 = 2. Step 4: Simplify: (8 + x) / 3 = 2 ⇒ 8 + x = 6 ⇒ x = 6 − 8 = −2. Step 5: Apply the centroid formula for the y coordinate: (yA + yB + yC) / 3 = 2. Step 6: Substitute yA = −1, yB = 2 and yC = y: (−1 + 2 + y) / 3 = 2. Step 7: Simplify: (1 + y) / 3 = 2 ⇒ 1 + y = 6 ⇒ y = 6 − 1 = 5. Step 8: Therefore, vertex C has coordinates (−2, 5). Verification / Alternative check: Check the centroid calculation explicitly. Average of x coordinates: (7 + 1 + (−2)) / 3 = (6) / 3 = 2. Average of y coordinates: (−1 + 2 + 5) / 3 = 6 / 3 = 2. Both match the given centroid (2, 2), confirming that our computed point C(−2, 5) is correct. Why Other Options Are Wrong: (2, 5), (−2, −5), (2, −5) and (0, 5) all fail when substituted into the centroid formula; the resulting average coordinates are not (2, 2). For each incorrect option, either the x average or the y average (or both) deviate from 2, so they cannot represent the true position of vertex C consistent with the given centroid. Common Pitfalls: A typical mistake is to mix up the formula, for example dividing by 2 instead of 3, or averaging only two vertices. Another common issue is arithmetic error when solving the simple linear equations, especially with negative numbers. Writing down the equations carefully and checking each substitution helps avoid such slips. Final Answer: The coordinates of vertex C are (−2, 5).

Question 20

The straight line ax − 4y = −6 has slope −3/2. What is the value of the coefficient a?

Accepted Answer

Introduction / Context: This question tests your understanding of how to read the slope of a line from its equation in general form. Converting a linear equation to slope intercept form y = mx + c makes it easy to identify the slope m and relate it to given information. This is a core skill in coordinate geometry. Given Data / Assumptions: The equation of the line is ax − 4y = −6. The slope of this line is given as −3 / 2. We must find the value of a that makes the slope equal to −3 / 2. We assume the usual slope intercept interpretation: slope is the coefficient of x when the equation is written as y = mx + c. Concept / Approach: To find the slope, we rewrite the line in the form y = mx + c. The coefficient m of x is the slope. Starting from ax − 4y = −6, solve for y in terms of x. The resulting expression will involve a, and equating its coefficient of x to −3 / 2 will allow us to solve for a. This is straightforward algebra once the equation is rearranged correctly. Step-by-Step Solution: Step 1: Start with the given equation ax − 4y = −6. Step 2: Isolate the y term: −4y = −ax − 6. Step 3: Divide both sides by −4 to solve for y: y = (−ax − 6) / (−4) = (ax + 6) / 4. Step 4: Rewrite in the form y = mx + c: y = (a / 4)x + 6 / 4. Step 5: The slope m of the line is therefore a / 4. Step 6: We are told the slope is −3 / 2, so set a / 4 = −3 / 2. Step 7: Solve for a: multiply both sides by 4 to get a = 4 * (−3 / 2) = −12 / 2 = −6. Verification / Alternative check: Substitute a = −6 back into the equation to check the slope. The line becomes −6x − 4y = −6. Rearranging, −4y = 6x − 6, so y = (−6 / 4)x + (6 / 4) = (−3 / 2)x + 3 / 2. The coefficient of x is indeed −3 / 2, confirming that the slope condition is satisfied and that a = −6 is correct. Why Other Options Are Wrong: If a = 6, 3, −3 or 0, the slope becomes a / 4 = 3 / 2, 3 / 4, −3 / 4 or 0 respectively, none of which equal −3 / 2. Each alternative value for a changes the tilt of the line, so these options do not match the given slope requirement. Common Pitfalls: Errors often come from sign mistakes when moving terms across the equal sign or dividing by negative numbers. Another common issue is misreading the general form and thinking the slope is directly a or −a / 4 without performing the proper algebra. Always explicitly rearrange to y = mx + c to avoid confusion and double check the sign of each term. Final Answer: The value of a is −6.

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