(z²-x²-y²)-4x²y²=(z²-x²-y²-2xy)(z²-x²-y²+2xy)=[z²-(x²+y²+2xy)][z²-(x²+y²-2xy)]=[z²-(x+y)²][z²-(x-y)&...(z²-x²-y²)²-4x²y²(z2-x2-y2)2-4x2y2就是（Z平方-x平方-y平方）-4x平方y平方