（x-y+z）²-(x+y-z)²
=[(x-y+z)+(x+y-z)][(x-y+z)-(x+y-z)]
=(x-y+z+x+y-z)(x-y+z-x-y+z)
=2x × (-2y+2z)
=-4xy+4xz
=-4x(y-z)