（x+y)(x-y)-(x+y)^2
=(x+y)[(x-y)-(x+y)]
=(x+y)(x-y-x-y)
=-2y(x+y)第一步什么意思啊？回答对了，采纳为最佳答案（x+y)(x-y)-(x+y)^2=(x+y)[(x-y)-(x+y)]…………提取公因式（x+y)=(x+y)(x-y-x-y)=-2y(x+y)