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