If $\sqrt{1 + \frac{55}{729}} = 1 + \frac{x}{27}$, then the value of $x$ is
Aptitude
Square Root and Cube Root
Difficulty: Medium
Choose an option
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A1
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B3
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C5
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D7
Answer
Correct Answer: 1
Explanation
Concept & Formula
To solve equations with variables outside a radical, first simplify the numeric expression under the square root, compute its exact root, and then algebraically isolate the unknown variable.
Step-by-Step Solution
* Given equation: $\sqrt{1 + \frac{55}{729}} = 1 + \frac{x}{27}$
* Simplify the expression under the square root by finding a common denominator:
$1 + \frac{55}{729} = \frac{729 + 55}{729} = \frac{784}{729}$
* The equation becomes: $\sqrt{\frac{784}{729}} = 1 + \frac{x}{27}$
* Extract the square root of the numerator and the denominator. We know that $27^2 = 729$. For $784$, since $25^2 = 625$ and $30^2 = 900$, and it ends in $4$, the root is $28$.
$\frac{\sqrt{784}}{\sqrt{729}} = \frac{28}{27}$
* Substitute this back into the equation: $\frac{28}{27} = 1 + \frac{x}{27}$
* Subtract $1$ from both sides: $\frac{28}{27} - 1 = \frac{x}{27}$
* $\frac{28 - 27}{27} = \frac{x}{27}$
* $\frac{1}{27} = \frac{x}{27}$
* Since the denominators are equal, the numerators must be equal: $x = 1$
Exam Strategy & Shortcut
Recognize the pattern $\sqrt{1 + \frac{k}{d^2}} = \frac{n}{d}$. Here, the denominator is $d = 27$ ($27^2 = 729$). The numerator under the root becomes $729 + 55 = 784$. The root is $\frac{28}{27}$. The right side is structured as $1 + \frac{x}{27} = \frac{27 + x}{27}$. Equating numerators: $28 = 27 + x$, which instantly gives $x = 1$.
Common Pitfall
Students often try to square both sides of the initial equation right away, resulting in $1 + \frac{55}{729} = (1 + \frac{x}{27})^2$. Expanding the right side creates a quadratic equation $(1 + \frac{2x}{27} + \frac{x^2}{729})$, making the math unnecessarily tedious and increasing the risk of calculation errors.
Final Answer
Therefore, the correct answer is **1**.