1)
$\left \{ {{2x+y=4} \atop {x-y=5}} \right.$
⇔ $\left \{ {{3x=9} \atop {x-y=5}} \right.$
⇔$\left \{ {{x=3} \atop {y=-2}} \right.$
2)
$\left \{ {{2x-y=1} \atop {x+3y=11}} \right.$
⇔$\left \{ {{2x-y=1} \atop {2x+6y=22}} \right.$
⇔$\left \{ {{-7y=-21} \atop {2x-y=1}} \right.$
⇔$\left \{ {{y=3} \atop {x=2}} \right.$
Giải thích các bước giải: