>>174
わかんなけりゃ力技

(x+y+z)(yz+zx+xy)-xyz
=xyz+zx^2+x^2y+y^2z+xyz+xy^2+yz^2+z^2x+xyz-xyz
=(y+z)x^2+(y^2+2yz+z^2)x+yz(y+z)
=(y+z)x^2+(y+z)^2x+yz(y+z)
=(y+z){x^2+(y+z)x+yz}
=(y+z)(z+x)(x+y)

そんな手間ちゃうで