x=-2011
y=-2
(x-y)(x+2y)-(x-2y)(x+y)
=xx -xy+2xy-2yy -(xx-2xy+xy-2yy)
=xx +xy -2yy -xx+2xy-xy+2yy
=2xy
=2*-2011*-2
=8044请问你算错了没有 xx -xy+2xy-2yy -(xx-2xy+xy-2yy)
是 =xx -xy+2xy-2yy -xx+2xy -xy +2yy 你都没有变号=xx -xy+2xy-2yy -(xx-2xy+xy-2yy)
=xx -xy+2xy-2yy-xx+2xy-xy+2yy
=xx +xy -2yy -xx+2xy-xy+2yy这里合并的时候蹦了一步,直接合并了第二项和第三项.没有错的.