# If X = a (1 + Cos θ), Y = A(θ + Sin θ), Prove that D 2 Y D X 2 = − 1 a A T θ = π 2 - Mathematics

Sum

If x = a (1 + cos θ), y = a(θ + sin θ), prove that $\frac{d^2 y}{d x^2} = \frac{- 1}{a}at \theta = \frac{\pi}{2}$

#### Solution

Here,

$x = a\left( 1 + \cos\theta \right) \text{ and } y = a\left( \theta + \sin\theta \right)$

$\text{ Differentiating w . r . t .} \theta, \text{ we get }$

$\frac{d x}{d \theta} = - a\sin\theta \text{ and } \frac{d y}{d \theta} = a + a \cos\theta$

$\therefore \frac{d y}{d x} = \frac{a + a\cos\theta}{- a\sin\theta} = \frac{1 + \cos\theta}{- \sin\theta}$

$\text{ Differentiating w . r . t . x, we get }$

$\frac{d^2 y}{d x^2} = \frac{d}{d\theta}\left\{ \frac{d y}{d x} \right\}\frac{d\theta}{dx}$

$\frac{d^2 y}{d x^2} = - \left\{ \frac{- \sin^2 \theta - \cos\theta - \cos^2 \theta}{\sin^2 \theta} \right\}\frac{d\theta}{dx}$

$= \frac{1 + \cos\theta}{\sin^2 \theta} \times \frac{- 1}{a \sin\theta}$

$= \frac{- \left( 1 + \cos\theta \right)}{a \sin^3 \theta}$

$\text{ At } \theta = \frac{\pi}{2}: \frac{d^2 y}{d x^2} = \frac{- \left( 1 + \cos\frac{\pi}{2} \right)}{a \left( \sin\frac{\pi}{2} \right)^3} = \frac{- 1}{a}$

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#### APPEARS IN

RD Sharma Class 12 Maths
Chapter 12 Higher Order Derivatives
Exercise 12.1 | Q 16 | Page 17