a) Điều kiện xác định:
$\begin{array}{l} 2 - \sin 6x > 0 \Leftrightarrow \sin 6x < 2\left( {\text{Luôn đúng}} \right)\\ \Rightarrow D = \mathbb{R} \end{array}$
b)
$\begin{array}{l} \left\{ \begin{array}{l} \sin 2x \ne 0\\ \cos x + 1 \ne 0 \end{array} \right. \Leftrightarrow \left\{ \begin{array}{l} \sin 2x \ne 0\\ \cos x \ne - 1 \end{array} \right. \Leftrightarrow \left\{ \begin{array}{l} 2x \ne k\pi \\ x \ne \pi + k2\pi \end{array} \right.\\ \Rightarrow x \ne \dfrac{{k\pi }}{2}\left( {k \in Z} \right)\\ D = R\backslash \left\{ {\dfrac{{k\pi }}{2},k \in Z} \right\} \end{array}$
c) Điều kiện xác định
$\begin{array}{l} \left\{ \begin{array}{l} \cos 2x \ne 0\\ \sin x \ne 0 \end{array} \right. \Leftrightarrow \left\{ \begin{array}{l} 2x \ne \dfrac{\pi }{2} + k\pi \\ x \ne k\pi \end{array} \right. \Leftrightarrow \left\{ \begin{array}{l} x \ne \dfrac{\pi }{4} + \dfrac{{k\pi }}{2}\\ x \ne k\pi \end{array} \right.\\ \Rightarrow D = \mathbb{R}\backslash \left\{ {\dfrac{\pi }{4} + \dfrac{{k\pi }}{2},k\pi |k \in Z} \right\} \end{array}$
