$$\eqalign{
& y = \sqrt {{{1 - \sin x} \over {1 + \cos x}}} \cr
& DKXD:\,\,\left\{ \matrix{
{{1 - \sin x} \over {1 + \cos x}} \ge 0\,\,\left( 1 \right) \hfill \cr
1 + \cos x \ne 0\,\,\left( 2 \right) \hfill \cr} \right. \cr
& \sin x \le 1 \Leftrightarrow \sin x - 1 \le 0 \cr
& \cos x \ge - 1 \Leftrightarrow \cos x + 1 \ge 0 \cr
& \Rightarrow {{\sin x - 1} \over {\cos x + 1}} \le 0 \cr
& \left( 1 \right) \Rightarrow {{\sin x - 1} \over {\cos x + 1}} = 0 \Leftrightarrow \sin x = 1 \Leftrightarrow x = {\pi \over 2} + k2\pi \cr
& \left( 2 \right) \Leftrightarrow \cos x \ne - 1 \Leftrightarrow x \ne \pi + k2\pi \cr
& Vay\,\,D = \left\{ {{\pi \over 2} + k2\pi ;\,\,k \in Z} \right\} \cr} $$
