ĐKXĐ:
a.
\(sin3x-sinx\ne0\)
\(\Leftrightarrow sin3x\ne sinx\Leftrightarrow\left\{{}\begin{matrix}3x\ne x+k2\pi\\3x\ne\pi-x+k2\pi\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}x\ne k\pi\\x\ne\frac{\pi}{4}+\frac{k\pi}{2}\end{matrix}\right.\)
b.
\(cos3x-cosx\ne0\Leftrightarrow cos3x\ne cosx\)
\(\Leftrightarrow\left[{}\begin{matrix}3x\ne x+k2\pi\\3x\ne-x+k2\pi\end{matrix}\right.\) \(\Leftrightarrow\left[{}\begin{matrix}x\ne k\pi\\x\ne\frac{k\pi}{2}\end{matrix}\right.\) \(\Leftrightarrow x\ne\frac{k\pi}{2}\)
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