\(\left\{{}\begin{matrix}x-y=1\\3x+y=7\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=1+y\\3\left(1+y\right)+y=7\end{matrix}\right.\)
\(\Leftrightarrow\left\{{}\begin{matrix}x=1+y\\3+3y+y=7\end{matrix}\right.\Leftrightarrow\left\{{}\begin{matrix}x=2\\y=1\end{matrix}\right.\)
\(\left\{{}\begin{matrix}x-y=1\left(1\right)\\3x+y=7\left(2\right)\end{matrix}\right.\)
nhân vế `1` với `3`
`=>`\(\left\{{}\begin{matrix}3x-3y=3\left(3\right)\\3x+y=7\left(2\right)\end{matrix}\right.\)
trừ `3` với `2` có :
`3x - 3y - 3x -y = 3 - 7`
`=> -4y = -4`
`=> y = -4 : -4`
`=> y = 1`
thay `y = 1` vào `1` ta đc
`x - 1 = 1`
`=> x = 1+ 1`
`=> x =2`
Vậy `(x,y) = (2,1)`
