（x+y-z）（x-y+z）-（x+y+z）（x-y-z）
=(x+y-z)[x-(y-z)]-(x+y+z)[x-(y+z)]
=[x²-(y-z)²]-[x²-(y+z)²]
=x²-(y-z)²-x²+(y+z)²
=(y+z)²-(y-z)²
=(y+z+y-z)(y+z-y+z)
=2y2z
=4yz