x^2y-y^2z+xz^2-x^2z+xy^2+yz^2-2xyz=（x² y-x² z）+(xz²+xy²-2xyz)-(y² z-yz²)=x²(y-z)+x(y²-2yz+z²)-yz(y-z)=x²(y-z)+x(y-z)²-yz(y-z)=(y-z)[x²+x(y-z)-yz]=(y-z...