已知x+y+z=0 化简x^2-y^2+xz-yz x^2-y^2+xz-yz = （x+y）(x-y)+z(x-y) =(x-y)(x+y+z) =0
原式=(x+y)(x-y)+z(x-y)=(x-y)(x+y+z)=0
x^2-y^2+xz-yz=(x+y)(x-y)+z(x-y)=(x-y)(x+y+z)=0
