(xy+z)(-xy+z)=(z+xy)(z-xy)=z²-(xy)²=z²-x²y²
你好：(xy+z)(-xy+z)=xy(-xy+z)+z(-xy+z)=-xyxy+xyz+xyz+z^2=-2xy+2xyz+z^2
zˇ2-xˇ2yˇ2
