a/ \(\left\{{}\begin{matrix}tanx\ne-1\\cosx\ne0\end{matrix}\right.\) \(\Leftrightarrow\left\{{}\begin{matrix}x\ne-\frac{\pi}{4}+k\pi\\x\ne\frac{\pi}{2}+k\pi\end{matrix}\right.\)
b/ \(\left\{{}\begin{matrix}sinx\ne0\\cosx\ne0\\tanx\ne1\end{matrix}\right.\) \(\Leftrightarrow\left\{{}\begin{matrix}sin2x\ne0\\tanx\ne1\end{matrix}\right.\) \(\Leftrightarrow\left\{{}\begin{matrix}x\ne\frac{k\pi}{2}\\x\ne\frac{\pi}{4}+k\pi\end{matrix}\right.\)