Đáp án:
4) \(x \ne k\pi \)
Giải thích các bước giải:
\(\begin{array}{l}
1)DK:\sin x - 1 \ne 0\\
\to \sin x \ne 1\\
\to x \ne \dfrac{\pi }{2} + k2\pi \left( {k \in Z} \right)\\
2)DK:\cos x \ne 0\\
\to x \ne \dfrac{\pi }{2} + k\pi \left( {k \in Z} \right)\\
3)\cos x \ne 0\\
\to x \ne \dfrac{\pi }{2} + k\pi \left( {k \in Z} \right)\\
4)DK:\cos 2x - 1 \ne 0\\
\to \cos 2x \ne 1\\
\to 2x \ne k2\pi \\
\to x \ne k\pi \left( {k \in Z} \right)
\end{array}\)
