原式=[（y+z）+x][（y+z）-x][x-（y-z）][x+（y-z）]
=[（y+z）²-x²][x²-（y-z）²]
=（y²+2yz+z²-x²）（x²-y²-z²+2yz）
=[2yz+（y²+z²-x²）][2yz-（y²+z²-x²）]
=（2yz）²-（y²+z²-x²）²
=4y²z²-y⁴-z⁴-x⁴-2y²z²+2x²y²+2x²z²
=-x⁴-y⁴-z⁴+2y²z²+2x²y²+2x²z²．