1/2(x+y+z)的平方+1/2(x-y-z)(x-y+z)-z(x+y)化简为多少?

1/2(x+y+z)的平方+1/2(x-y-z)(x-y+z)-z(x+y)化简为多少?
1/2(x+y+z)的平方+1/2(x-y-z)(x-y+z)-z(x+y)化简为多少?

1/2(x+y+z)的平方+1/2(x-y-z)(x-y+z)-z(x+y)化简为多少?
1/2(x+y+z)的平方+1/2(x-y-z)(x-y+z)-z(x+y)
=½(x²+y²+z²+2xy+2xz+2yz)+½(x²-2xy+y²-z²)-zx-zy
=½x²+½y²+½z²+xy+xz+yz+½x²-xy+½y²-½z²-zx-zy
=x²+y²

我也不知到！！！