ĐKXĐ:
a. \(x^2-9\ne0\Rightarrow x\ne\pm3\) hay \(D=R\backslash\left\{-3;3\right\}\)
b. \(\left\{{}\begin{matrix}1-x^2\ne0\\-x\ge0\end{matrix}\right.\) \(\Rightarrow\left\{{}\begin{matrix}x\le0\\x\ne-1\end{matrix}\right.\) hay \(D=(-\infty;0]\backslash\left\{-1\right\}\)
c. \(\left\{{}\begin{matrix}2-x\ge0\\x+2>0\end{matrix}\right.\) \(\Rightarrow D=(-2;2]\)