问题描述: 已知x+y=7,x-y=5,求x2-y2-2y+2x的值. 1个回答 分类:数学 2014-11-07 问题解答: 我来补答 x2-y2-2y+2x=(x+y)(x-y)-2(y-x)=(x+y)(x-y)+2(x-y)=(x-y)(x+y+2),将x+y=7,x-y=5,代入上式得:原式=(x-y)(x+y+2)=5×9=45. 展开全文阅读