原式=x(y-z)-(y²-2yz+z²)=x(y-z)-(y-z)²=(y-z)(x-y+z)
原式=x(y-z)-(y-z)^2=(y-z)(x-y+z)
xy-xz-y^2+2yz-z^2=x(y-z)-(y-z)²=（y-z）(x-y+z)
