(x+y)²-(x-y)²=[(x+y)+(x-y)][(x+y)-(x-y)]=(x+y+x-y)(x+y-x+y)=2x*2y=4xy希望采纳
(x+y)^2-(x-y)^2=[(x+y)+(x-y)].[(x+y)-(x-y)]=4xy
（x+y+x-y)(x+y-x+y)=4xy
