两个方程式左边相加，右边也想加：x-y+2x+y=4+5=9 3x=9 x=3
x-y=4,2x+y=5x-y+2x+y=5+43x=9 x=33-y=4y=3-4=-1
①式+②式得3x=9 x=3 带入①式y=-1
