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NDA 2025
Exam
NDA
Year
2025
Subject
Mathematics
Chapter
Application of Derivatives

NDA 2025 · Mathematics · Application of Derivatives

For the following three (03) items: Let y = f(x) = x ^ -1 x 1 - x^2...

For the following three (03) items:
Let y=f(x)=xsin⁡−1x1−x2+ln⁡1−x2y = f(x) = \dfrac{x\sin^{-1} x}{\sqrt{1 - x^2}} + \ln\sqrt{1 - x^2}.

What is the slope of the tangent to the curve y=f(x)y = f(x) at x=0.5x = 0.5?

Options

  1. A4π3/274\pi\sqrt{3}/27
  2. B8π3/278\pi\sqrt{3}/27
  3. C4π4\pi
  4. D8π8\pi

Correct answer

A. 4π3/274\pi\sqrt{3}/27

Step-by-step solution

Given y=f(x)=xsin⁡−1x1−x2+ln⁡1−x2y = f(x) = \dfrac{x\sin^{-1} x}{\sqrt{1 - x^2}} + \ln\sqrt{1 - x^2}

We can rewrite the function as:
y=xsin⁡−1x1−x2+12ln⁡(1−x2)y = \dfrac{x\sin^{-1} x}{\sqrt{1 - x^2}} + \dfrac{1}{2}\ln(1 - x^2)

Differentiating with respect to xx using the product rule and chain rule:
dydx=ddx(x1−x2)sin⁡−1x+x1−x2ddx(sin⁡−1x)+1211−x2(−2x)\dfrac{dy}{dx} = \dfrac{d}{dx}\left(\dfrac{x}{\sqrt{1 - x^2}}\right)\sin^{-1} x + \dfrac{x}{\sqrt{1 - x^2}}\dfrac{d}{dx}(\sin^{-1} x) + \dfrac{1}{2}\dfrac{1}{1 - x^2}(-2x)

dydx=[1⋅1−x2−x⋅(−x1−x2)1−x2]sin⁡−1x+x1−x2⋅11−x2−x1−x2\dfrac{dy}{dx} = \left[\dfrac{1\cdot\sqrt{1 - x^2} - x\cdot\left(\dfrac{-x}{\sqrt{1 - x^2}}\right)}{1 - x^2}\right]\sin^{-1} x + \dfrac{x}{\sqrt{1 - x^2}}\cdot\dfrac{1}{\sqrt{1 - x^2}} - \dfrac{x}{1 - x^2}

dydx=[1−x2+x2(1−x2)3/2]sin⁡−1x+x1−x2−x1−x2\dfrac{dy}{dx} = \left[\dfrac{1 - x^2 + x^2}{(1 - x^2)^{3/2}}\right]\sin^{-1} x + \dfrac{x}{1 - x^2} - \dfrac{x}{1 - x^2}

dydx=sin⁡−1x(1−x2)3/2\dfrac{dy}{dx} = \dfrac{\sin^{-1} x}{(1 - x^2)^{3/2}}

To find the slope of the tangent at x=0.5=12x = 0.5 = \dfrac{1}{2}, we substitute this value into the derivative:
dydx∣x=1/2=sin⁡−1(1/2)(1−(1/2)2)3/2\dfrac{dy}{dx}\bigg|_{x=1/2} = \dfrac{\sin^{-1}(1/2)}{(1 - (1/2)^2)^{3/2}}

dydx∣x=1/2=π/6(3/4)3/2=π/633/8\dfrac{dy}{dx}\bigg|_{x=1/2} = \dfrac{\pi/6}{(3/4)^{3/2}} = \dfrac{\pi/6}{3\sqrt{3}/8}

dydx∣x=1/2=8π183=4π93\dfrac{dy}{dx}\bigg|_{x=1/2} = \dfrac{8\pi}{18\sqrt{3}} = \dfrac{4\pi}{9\sqrt{3}}

Rationalizing the denominator by multiplying the numerator and denominator by 3\sqrt{3}:
dydx∣x=1/2=4π327\dfrac{dy}{dx}\bigg|_{x=1/2} = \dfrac{4\pi\sqrt{3}}{27}
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