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Provide solution for RD Sharma maths class 12 chapter 10 Differentiation exercise Very short answers question 14

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Answer:

\text { The value of } \frac{d y}{d x}=-\tan \theta / 2

Given:

\text { If } x=a(\theta+\sin \theta), y=a(1+\cos \theta) \text { find } \frac{d y}{d x}

Hint:

\begin{aligned} &\frac{d}{d x}(\sin x)=\cos x \\\\ &\frac{d}{d x}(\cos x)=-\sin x \end{aligned}

Solution:  

\begin{aligned} &\frac{d x}{d \theta}=a \frac{d}{d \theta}(\theta+\sin \theta) \\\\ &=a[1+\cos \theta] \end{aligned}

\begin{aligned} &\frac{d y}{d \theta}=a \frac{d}{d \theta}[1+\cos \theta] \\\\ &=-a \sin \theta \end{aligned}

\begin{aligned} &\frac{d y}{d x}=\frac{-a \sin \theta}{a(1+\cos \theta)} \\\\ &=\frac{-\sin \theta}{1+\cos \theta} \end{aligned}

=\frac{-2 \sin ^{\theta} / 2 \cos ^{\theta} / 2}{2 \cos ^{2} \theta / 2}        using formula of half angle

=-\tan \theta / 2

So, the answer of  \frac{d y}{d x}=-\tan \theta / 2

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