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NDA 2025
Exam
NDA
Year
2025
Subject
Mathematics
Chapter
Differentiation

NDA 2025 · Mathematics · Differentiation

Consider the following for the two (02) items that follow: Let...

Consider the following for the two (02) items that follow:
Let (x+y)p+q=xpyq(x+y)^{p+q} = x^p y^q, where pp, qq are positive integers.

The derivative of yy with respect to xx

Options

  1. Adepends on pp only
  2. Bdepends on qq only
  3. Cdepends on both pp and qq
  4. Dis independent of both pp and qq

Correct answer

D. is independent of both pp and qq

Step-by-step solution

Given (x+y)p+q=xpyq(x+y)^{p+q} = x^p y^q

Taking the natural logarithm on both sides:

(p+q)ln⁡(x+y)=pln⁡x+qln⁡y(p+q) \ln(x+y) = p \ln x + q \ln y

Differentiating both sides with respect to xx:

p+qx+y(1+dydx)=px+qydydx\dfrac{p+q}{x+y} \left(1 + \dfrac{dy}{dx}\right) = \dfrac{p}{x} + \dfrac{q}{y} \dfrac{dy}{dx}

Rearranging the terms to solve for dydx\dfrac{dy}{dx}:

p+qx+y−px=(qy−p+qx+y)dydx\dfrac{p+q}{x+y} - \dfrac{p}{x} = \left( \dfrac{q}{y} - \dfrac{p+q}{x+y} \right) \dfrac{dy}{dx}

Simplifying the left side:

x(p+q)−p(x+y)x(x+y)=px+qx−px−pyx(x+y)=qx−pyx(x+y)\dfrac{x(p+q) - p(x+y)}{x(x+y)} = \dfrac{px + qx - px - py}{x(x+y)} = \dfrac{qx - py}{x(x+y)}

Simplifying the right side:

q(x+y)−y(p+q)y(x+y)=qx+qy−py−qyy(x+y)=qx−pyy(x+y)\dfrac{q(x+y) - y(p+q)}{y(x+y)} = \dfrac{qx + qy - py - qy}{y(x+y)} = \dfrac{qx - py}{y(x+y)}

Equating the two sides:

qx−pyx(x+y)=qx−pyy(x+y)dydx\dfrac{qx - py}{x(x+y)} = \dfrac{qx - py}{y(x+y)} \dfrac{dy}{dx}

Canceling the common terms from both sides:

1x=1ydydx\dfrac{1}{x} = \dfrac{1}{y} \dfrac{dy}{dx}

dydx=yx\dfrac{dy}{dx} = \dfrac{y}{x}

The derivative dydx\dfrac{dy}{dx} is yx\dfrac{y}{x}, which is independent of both pp and qq.

Answer: is independent of both pp and qq
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