\(\left\{{}\begin{matrix}x+y=m\\2x+my=2m+2\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}2x+2y=2m\\2x+my=2m+2\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}\left(2-m\right)y=-2\\x+y=m\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}y=\dfrac{2}{m-2}\\x=\dfrac{m^2-2m-2}{m-2}\end{matrix}\right.\)
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