Directions: Solve the quadratic and answer the following questions. A : $(x-2)^2 = (-3x^2) + 2^2 + 25x - P$ B: $\left(10y^2 - 3^2y + \frac{2}{3}\right) \times 3 + 10y = 0$ One root of equation A is $5$. Find the product of smallest root of equation A and smallest two-digit prime number.
Aptitude
Quadratic Equation
Difficulty: Medium
Choose an option
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A$13\frac{1}{4}$
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B$17\frac{1}{4}$
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C$24\frac{3}{4}$
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D$13\frac{3}{4}$
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E$23\frac{3}{7}$
Answer
Correct Answer: $24\frac{3}{4}$
Explanation
### Concept & Quadratic Roots
To solve this, we need to find all the roots of Equation A and identify the smallest one, then multiply it by the smallest two-digit prime number.
$$ \text{Product of roots} = \frac{c}{a} $$
### Step-by-Step Solution
1. **Find Equation A and its roots:**
From expanding and rearranging Equation A, we previously found:
$4x^2 - 29x + P = 0$
Substituting $x=5$, we found $P=45$.
The complete quadratic equation is: $4x^2 - 29x + 45 = 0$
2. **Solve for the second root:**
We can factorize the quadratic:
$4x^2 - 20x - 9x + 45 = 0$
$4x(x - 5) - 9(x - 5) = 0$
$(4x - 9)(x - 5) = 0$
The roots are $x = 5$ and $x = \frac{9}{4}$.
The smallest root of Equation A is $\frac{9}{4}$.
3. **Identify the smallest two-digit prime number:**
The prime numbers starting from single digits are $2, 3, 5, 7, 11, 13 \dots$
The smallest two-digit prime number is $11$.
4. **Calculate the final product:**
Product = $\frac{9}{4} \times 11 = \frac{99}{4}$
Convert $\frac{99}{4}$ into a mixed fraction:
$99 \div 4 = 24$ with a remainder of $3$.
So, $\frac{99}{4} = 24\frac{3}{4}$.
### Exam Strategy & Shortcut
Instead of factorizing, use the sum or product of roots. Product of roots = $\frac{c}{a} = \frac{45}{4}$. Since one root is $5$, the other is $\frac{45}{4} \div 5 = \frac{9}{4}$. This is significantly faster!
### Common Pitfall
A frequent error is misidentifying the smallest two-digit prime number as $10$ (which is composite) or $13$ (missing $11$), throwing off the final calculation.
### Final Answer
Therefore, the correct answer is **$24\frac{3}{4}$**.