\(a,\left\{{}\begin{matrix}3x+y=1\\x-2y=5\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}y=1-3x\\x-2.\left(1-3x\right)=5\end{matrix}\right.\) \(\Leftrightarrow\left\{{}\begin{matrix}y=1-3x\\x-2+6x=5\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}y=1-3x\\7x=7\end{matrix}\right.\) \(\Leftrightarrow\left\{{}\begin{matrix}y=1-3\\x=1\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}y=-2\\x=1\end{matrix}\right.\)
\(b,\left\{{}\begin{matrix}2x+y=8\\-x+y=2\end{matrix}\right.\) \(\Leftrightarrow\left\{{}\begin{matrix}y=8-2x\\-x+8-2x=2\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}y=8-2x\\-3x=-6\end{matrix}\right.\) \(\Leftrightarrow\left\{{}\begin{matrix}y=8-2.2\\x=2\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}y=4\\x=2\end{matrix}\right.\)
