(a)
$\frac{d}{dx} = \left( \tan^{- 1}\sqrt{\frac{1 + \cos\frac{x}{2}}{1 - \cos\frac{x}{2}}} \right)\ = \ \frac{d}{dx}\tan^{- 1}\sqrt{\frac{2\cos^{2}\frac{x}{4}}{2\sin^{2}\frac{x}{4}}}$
$= \frac{d}{dx}\ \tan^{- 1}\left| \cot\left( \frac{x}{4} \right) \right|$
$= \frac{d}{dx}\tan^{- 1}\left| \tan\left( \frac{\pi}{2} - \frac{x}{4} \right) \right|$
$= \frac{d}{dx}\left( \frac{\pi}{2} - \frac{x}{4} \right) = \frac{- 1}{4}$