ĐKXĐ: \(\left\{{}\begin{matrix}tan2x\ne0\\cos2x\ne0\end{matrix}\right.\) \(\Leftrightarrow\left\{{}\begin{matrix}sin2x\ne0\\cos2x\ne0\end{matrix}\right.\)
\(\Leftrightarrow sin2x.cos2x\ne0\)
\(\Leftrightarrow sin4x\ne0\)
\(\Leftrightarrow x\ne k\frac{\pi}{4}\)