(b)
Let $I = \int_{0}^{\pi/2}{\frac{\cos\theta}{\sqrt{4 - \sin^{2}\theta}}d\theta}$
Put $\sin{\theta = t\ \Rightarrow \ \cos{\theta d\theta = dt}}$
$\therefore\ I = \int_{0}^{1}\frac{dt}{\sqrt{4 - t^{2}}}$
$= \left\lbrack \sin^{- 1}\frac{t}{2} \right\rbrack_{0}^{1}$
$= \sin^{- 1}{\frac{1}{2} = \frac{\pi}{6}}$