ĐKXĐ: \(\left\{{}\begin{matrix}sin4x\ne0\\cos3x\ne0\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}sin4x\ne0\\4cos^3x-3cosx\ne0\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}sin4x\ne0\\4cos^2x-3\ne0\end{matrix}\right.\) \(\Leftrightarrow\left\{{}\begin{matrix}sin4x\ne0\\2cos2x-1\ne0\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}sin4x\ne0\\cos2x\ne\frac{1}{2}\end{matrix}\right.\) \(\Rightarrow\left\{{}\begin{matrix}x\ne\frac{k\pi}{4}\\x\ne\pm\frac{\pi}{6}+k\pi\end{matrix}\right.\)