a ) ĐKXĐ : \(\left\{{}\begin{matrix}x\ne0\\x\ne\pm3\end{matrix}\right.\)
\(A=\left(\frac{x^2-3}{x^2-9}+\frac{x+3}{x^2-9}\right)\cdot\frac{x+3}{x}\)
\(A=\frac{x^2+x}{\left(x-3\right)\left(x+3\right)}\cdot\frac{x+3}{x}\)
\(A=\frac{x\left(x+1\right)}{x-3}\cdot\frac{1}{x}=\frac{x+1}{x-3}\)
b) \(\left|A\right|=3\Leftrightarrow\left[{}\begin{matrix}A=3\\A=-3\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}\frac{x+1}{x-3}=3\\\frac{x+1}{x-3}=-3\end{matrix}\right.\) \(\Leftrightarrow\left[{}\begin{matrix}x+1=3x-9\\x+1=-3x+9\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}2x=10\\4x=8\end{matrix}\right.\Leftrightarrow\left[{}\begin{matrix}x=5\\x=2\end{matrix}\right.\) ( TM ĐKXĐ )