ĐKXĐ: \(\left\{{}\begin{matrix}cos3x\ne0\\sin4x\ne0\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}4cos^2x-3\ne0\\sin4x\ne0\end{matrix}\right.\) \(\Leftrightarrow\left\{{}\begin{matrix}cos2x\ne\frac{1}{2}\\sin4x\ne0\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}2x\ne\pm\frac{\pi}{3}+k2\pi\\4x\ne k\pi\end{matrix}\right.\) \(\Leftrightarrow\left\{{}\begin{matrix}x\ne\pm\frac{\pi}{6}+k\pi\\x\ne\frac{k\pi}{4}\end{matrix}\right.\)
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