(b)
$\sqrt{\frac{1 - \sin\theta}{1 + sin\ \theta}} + \sqrt{\frac{1 + \sin\theta}{1 - \sin\theta} =}\frac{1 - \sin{\theta + 1 + \sin\theta}}{\sqrt{1 - \sin^{2}\theta}}$
$= \frac{2}{\sqrt{\cos^{2}\theta}} = \frac{2}{|\cos\theta|}$
$= - \frac{2}{\cos\theta} = - 2\sec{\theta\left( \because\frac{\pi}{2} < \theta < \pi \right)}$