a.\(\left\{{}\begin{matrix}2x+y=1\\x-y=2\end{matrix}\right.\) \(\Leftrightarrow\left\{{}\begin{matrix}3x=3\\x-y=2\end{matrix}\right.\) \(\Leftrightarrow\left\{{}\begin{matrix}x=1\\1-y=2\end{matrix}\right.\) \(\Leftrightarrow\left\{{}\begin{matrix}x=1\\y=-1\end{matrix}\right.\)
b.\(A=\left(\dfrac{3}{\sqrt{x}-2}-\dfrac{\sqrt{x}}{x-2\sqrt{x}}\right):\dfrac{x}{\sqrt{x}-2}\)
\(A=\left(\dfrac{3}{\sqrt{x}-2}-\dfrac{\sqrt{x}}{\sqrt{x}\left(\sqrt{x}-2\right)}\right):\dfrac{x}{\sqrt{x}-2}\)
\(A=\left(\dfrac{3}{\sqrt{x}-2}-\dfrac{1}{\sqrt{x}-2}\right).\dfrac{\sqrt{x}-2}{x}\)
\(A=\dfrac{2}{\sqrt{x}-2}.\dfrac{\sqrt{x}-2}{x}\)
\(A=\dfrac{2}{x}\)
