x + 2y = -3 ; 3x + y = 1

$\begin{cases} x + 2y = -3\text{ }\text{ }\text{ }\text{ }\text{ }\text{ }\text{ }\text{ }\text{ }\text{ }\text{ }\text{ (*)}\\3x + y = 1 \end{cases}$

`<=>` $\begin{cases} 3x + 6y = -9\text{ }\text{ }\text{ }\text{ }\text{ }\text{ }\text{ }\text{ }\text{ }\text{ }\text{ }\text{ (1)}\\3x + y = 1 \text{ }\text{ }\text{ }\text{ }\text{ }\text{ }\text{ }\text{ }\text{ }\text{ }\text{ }\text{ }\text{ }\text{ }\text{ }\text{ }\text{ (2)}\end{cases}$

`(1-2)` `=>` `5y` `=` `-10`

`<=>` `y` `=` `-2`

$\text{Thay}$ `y` `=` `-2` $\text{vào}$ $\text{(*)}$

`=>` `x` `+` `2` `.` `(-2)` `=` `-3`

`<=>` `x` `-` `4` `=` `-3`

`<=>` `x` `=` `1`

`=>` $\begin{cases} x=1\\y = -2 \end{cases}$

`#TranDucQuangNBK`