`{(x+2y=5),(x-y=-1):}`
`<=>{(x+2y=5),(2x-2y=-2):}`
`<=>{(3x=3),(x-y=-1):}`
`<=>{(x=1),(1-y=-1):}`
`<=>{(x=1),(y=2):}`
trừ từng vế của pt ta đc:
\(\Leftrightarrow3x=6\)
\(\Leftrightarrow\left\{{}\begin{matrix}3x=6\\x-y=-1\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=2\\2-y=-1\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}x=2\\-y=-3\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=2\\y=3\end{matrix}\right.\)
vậy pt có nghiệm \(x=2;y=3\)
\(\left\{{}\begin{matrix}x=5-2y\\\left(5-2y\right)-y=-1\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=5-2y\\-3y=-6\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=5-2.2\\y=2\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=1\\y=2\end{matrix}\right.\)
