a/ ĐKXĐ:
\(cosx-1\ne0\)
\(\Leftrightarrow cosx\ne1\)
\(\Rightarrow x\ne k2\pi\)
b/ ĐKXĐ: \(\left\{{}\begin{matrix}cosx\ne0\\cos\left(x-\frac{\pi}{4}\right)\ne0\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}x\ne\frac{\pi}{2}+k\pi\\x\ne\frac{3\pi}{4}+k\pi\end{matrix}\right.\)