Giải hệ phương trình sau :

$\left \{ {{2(x+y)+3(x-y)=4} \atop {(x+y)+2(x-y)=5}} \right.$

⇔$\left \{ {{2x+2y+3x-3y=4} \atop {x+y+2x-2y=5}} \right.$

⇔$\left \{ {{5x-y=4} \atop {3x-y=5}} \right.$

⇔$\left \{ {{5x-3x=4-5} \atop {3x-y=5}} \right.$

⇔$\left \{ {{2x=-1} \atop {3x-y=4}} \right.$

⇔$\left \{ {{x=\frac{-1}{2} } \atop {3*\frac{-1}{2}-y=4}} \right.$

⇔$\left \{ {{x=\frac{-1}{2} } \atop {y=\frac{-13}{2}}} \right.$