Đáp án:
$\begin{array}{l}
a)y = \sqrt {2 - {\mathop{\rm s}\nolimits} {\rm{inx}}} \\
Dkxd:2 - \sin x \ge 0\\
\Leftrightarrow \sin x \le 2\left( {tm} \right)\\
Do:\sin x \le 1\forall x\\
Vậy\,TXD:D = R\\
b)y = \dfrac{2}{{\cos 2x}}\\
Dkxd:\cos 2x \ne 0\\
\Leftrightarrow 2x \ne \dfrac{\pi }{2} + k\pi \\
\Leftrightarrow x \ne \dfrac{\pi }{4} + \dfrac{{k\pi }}{2}\\
Vậy\,TXD:D = R\backslash \left\{ {\dfrac{\pi }{4} + \dfrac{{k\pi }}{2}} \right\}\\
c)y = \tan x + \cot x\\
Dkxd:\left\{ \begin{array}{l}
\cos x \ne 0\\
\sin x \ne 0
\end{array} \right.\\
\Leftrightarrow 2.\sin x.\cos x \ne 0\\
\Leftrightarrow \sin 2x \ne 0\\
\Leftrightarrow 2x \ne k\pi \\
\Leftrightarrow x \ne \dfrac{{k\pi }}{2}\\
Vậy\,TXD:D = R\backslash \left\{ {\dfrac{{k\pi }}{2}} \right\}\\
d)y = \dfrac{1}{{\sin x}} - \dfrac{1}{{2\cos x}}\\
Dkxd:\left\{ \begin{array}{l}
\sin x \ne 0\\
\cos x \ne 0
\end{array} \right. \Leftrightarrow x \ne \dfrac{{k\pi }}{2}\\
Vậy\,TXD:D = R\backslash \left\{ {\dfrac{{k\pi }}{2}} \right\}
\end{array}$
