KCET · Maths · Differential Equations
If \(\sqrt{x} \sqrt[3]{y} = (x + y)^n\) and \(x\dfrac{dy}{dx} - y = 0\), then \(n = \)
- A \(1\)
- B \(\dfrac{6}{5}\)
- C \(\dfrac{5}{6}\)
- D \(\dfrac{4}{9}\)
Answer & Solution
Correct Answer
(C) \(\dfrac{5}{6}\)
Step-by-step Solution
Detailed explanation
Given equation: \(\sqrt{x} \sqrt[3]{y} = (x + y)^n\)
Taking natural logarithm on both sides:
\(\dfrac{1}{2} \ln x + \dfrac{1}{3} \ln y = n \ln(x + y)\)
Differentiating with respect to \(x\):
\(\dfrac{1}{2x} + \dfrac{1}{3y} \dfrac{dy}{dx} = \dfrac{n}{x + y} \left(1 + \dfrac{dy}{dx}\right)\)
It is given that \(x \dfrac{dy}{dx} - y = 0 \Rightarrow \dfrac{dy}{dx} = \dfrac{y}{x}\)
Substituting \(\dfrac{dy}{dx} = \dfrac{y}{x}\) in the differentiated equation:
\(\dfrac{1}{2x} + \dfrac{1}{3y} \left(\dfrac{y}{x}\right) = \dfrac{n}{x + y} \left(1 + \dfrac{y}{x}\right)\)
\(\dfrac{1}{2x} + \dfrac{1}{3x} = \dfrac{n}{x + y} \left(\dfrac{x + y}{x}\right)\)
\(\dfrac{5}{6x} = \dfrac{n}{x}\)
\(n = \dfrac{5}{6}\)
Answer: \(\dfrac{5}{6}\)
Taking natural logarithm on both sides:
\(\dfrac{1}{2} \ln x + \dfrac{1}{3} \ln y = n \ln(x + y)\)
Differentiating with respect to \(x\):
\(\dfrac{1}{2x} + \dfrac{1}{3y} \dfrac{dy}{dx} = \dfrac{n}{x + y} \left(1 + \dfrac{dy}{dx}\right)\)
It is given that \(x \dfrac{dy}{dx} - y = 0 \Rightarrow \dfrac{dy}{dx} = \dfrac{y}{x}\)
Substituting \(\dfrac{dy}{dx} = \dfrac{y}{x}\) in the differentiated equation:
\(\dfrac{1}{2x} + \dfrac{1}{3y} \left(\dfrac{y}{x}\right) = \dfrac{n}{x + y} \left(1 + \dfrac{y}{x}\right)\)
\(\dfrac{1}{2x} + \dfrac{1}{3x} = \dfrac{n}{x + y} \left(\dfrac{x + y}{x}\right)\)
\(\dfrac{5}{6x} = \dfrac{n}{x}\)
\(n = \dfrac{5}{6}\)
Answer: \(\dfrac{5}{6}\)
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