Đáp án:
8C
Giải thích các bước giải:
\(\begin{array}{l}
C8:\\
DK:\cos x \ne 2\left( {ld} \right)\\
\to C\\
C9:\\
DK:\sin x \ne - 1\\
\to x \ne - \dfrac{\pi }{2} + k2\pi \left( {k \in Z} \right)\\
\to D\\
C10:\\
DK:1 - 2\sin x\cos x \ne 0\\
\to 1 - \sin 2x \ne 0\\
\to \sin 2x \ne 1\\
\to 2x \ne \dfrac{\pi }{2} + k2\pi \\
\to x \ne \dfrac{\pi }{4} + k\pi \left( {k \in Z} \right)\\
\to B\\
C11:\\
DK:\cos 3x \ne 0\\
\to 3x \ne \dfrac{\pi }{2} + k\pi \\
\to x \ne \dfrac{\pi }{6} + \dfrac{{k\pi }}{3}\left( {k \in Z} \right)
\end{array}\)
