因为x-5=y+2,x=y+7(y+2)的平方=y（y+7）,y2(平方)+4y+4=y2（平方）+7y,4y+4=7y,3y=4,y=3/4.
(x-5)(y+2)=xy xy-5y+2x-10=xy2x-5y=10 (1)x-5=y+2x-y=7 (2)(2)×5-(1)3x=25x=25/3y=x-7=4/3
