$\begin{array}{l}
y = \sqrt {\frac{{2 + \cos x}}{{1 - \sin 2x}}} \\
DK:\frac{{2 + \cos x}}{{1 - \sin 2x}} \ge 0\,\left( * \right)\\
Ta\,co:\,2 + \cos x > 0,\forall x\,(do\,\cos x \ge - 1\,nen\,2 + \cos x \ge 2 - 1 = 1)\\
Khi\,do\,\left( * \right) \Leftrightarrow 1 - \sin 2x > 0 \Leftrightarrow \sin 2x < 1 \Leftrightarrow \sin 2x \ne 1\,\left( {do\,\sin 2x \le 1,\forall x} \right)\\
\Leftrightarrow 2x \ne \frac{\pi }{2} + k2\pi \Leftrightarrow x \ne \frac{\pi }{4} + k\pi
\end{array}$
