(b)
On differentiating given curves w.r.t.$\theta$ respectively, we get
$\frac{dx}{d\theta} = a\left( - \sin\theta + \frac{1}{\tan\left( \frac{\theta}{2} \right)}\ .\sec^{2}\frac{\theta}{2}\ .\frac{1}{2} \right)\$
and $\frac{dy}{d\theta}$ = $a\cos\theta$
$\Rightarrow \ \frac{dx}{d\theta}$ = $\frac{a\cos^{2}\theta}{\sin\theta}\ and\ \frac{dy}{d\theta} = a\cos\theta$
$\therefore\ \ \ \frac{dy}{dx} = \frac{dy\ /d\theta}{dx/\ d\theta} = \frac{a\cos\theta}{a\cos^{2}{\theta/\sin\theta}}$= tan$\theta$