Đáp án:
d. \(x \ne \dfrac{{k\pi }}{2}\)
Giải thích các bước giải:
\(\begin{array}{l}
a.DK:\cos x \ne 0\\
\to x \ne \dfrac{\pi }{2} + k\pi \left( {k \in Z} \right)\\
b.DK:\cos \left( {5x + \dfrac{\pi }{3}} \right) \ne 0\\
\to 5x + \dfrac{\pi }{3} \ne \dfrac{\pi }{2} + k\pi \left( {k \in Z} \right)\\
\to 5x \ne \dfrac{\pi }{6} + k\pi \\
\to x \ne \dfrac{\pi }{{30}} + \dfrac{{k\pi }}{5}\left( {k \in Z} \right)\\
c.DK:\sin \left( {\dfrac{x}{2} + \dfrac{\pi }{5}} \right) \ne 0\\
\to \dfrac{x}{2} + \dfrac{\pi }{5} \ne k\pi \\
\to x \ne - \dfrac{{2\pi }}{5} + k2\pi \left( {k \in Z} \right)\\
d.DK:\sin 2x \ne 0\\
\to 2x \ne k\pi \\
\to x \ne \dfrac{{k\pi }}{2}\left( {k \in Z} \right)
\end{array}\)
