原式＝(x+y)(x-y)-(x+y)^2 =(x+y)(x-y-(x+y)) =-2y(x+y)
x*x-y*y-(x+y)(x+y)=(x-y)(x+y)-(x+y)(x+y)=(x+y)[(x-y)-(x+y)]=(x+y)(-2y)=-2y(x+y)
原式=(x+y)(x-y)-(x+y)(x+y)=-2y(x+y)
