$\left \{ {{x+y=12} \atop {x+2y=17}} \right.$ <=>$\left \{ {{2x+2y=24} \atop {x+2y=17}} \right.$ <=> $\left \{ {{x=7} \atop {x+y=12}} \right.$ <=>$\left \{ {{x=7} \atop {7+y=12}} \right.$ => $\left \{ {{x=7} \atop {y=5}} \right.$
Kết luận : HPT có nghiệm : $\left \{ {{y=5} \atop {x=7}} \right.$