x4+y4+z4-2x2y2-2y2z2-2z2x2．

原式=x4-2x2y2+y4-2y2z2-2z2x2+z4=（x2-y2）2-2z2（x2+y2）+z4=（x2-y2）2-2z2（x2-y2）+z4-4x2y2=（x2-y2-z2）2-4x2y2=（x2-y2-z2-2yz）（x2-y2-z2+2yz）=[x2-（y+z）2][x2-（y-z）2]=（x+y+z）（x-y-z）（x+y-z）...