ĐKXĐ: \(\left\{ \begin{array}{l}\cos x \ne 0\\1 - \sin x > 0\end{array} \right. \)
\(\Leftrightarrow \left\{ \begin{array}{l}\cos x \ne 0\\1 - \sin x \ne 0\end{array} \right. \)
\(\Leftrightarrow \left\{ \begin{array}{l}x \ne \dfrac{\pi }{2} + k\pi\\x \ne \dfrac{\pi }{2} + k2\pi\end{array} \right. \)
\(\Leftrightarrow x \ne \dfrac{\pi }{2} + k\pi \)
Vậy \(x \ne \dfrac{\pi }{2} + k\pi ,k \in \mathbb{Z}\) hay \(\mathbb{R}\backslash \left\{ {\dfrac{\pi }{2} + k\pi ,k \in \mathbb{Z}} \right\}\)
Đáp án: C.
