Rút gọn bt `(x-y)(y+z)(z+x)+(x+y)(y-z)(z+x)+(x+y)(y+z)(z-x)`

`(x-y)(y+z)(z+x)+(x+y)(y-z)(z+x)+(x+y)(y+z)(z-x)`

`= (xy+xz+y^2 -yz)(z+x)+ (xy-xz+y^2 -yz)(z+x)+(xy+xz+y^2 +yz)(z-x)`

`= xyz+x^2 y+xz^2 +x^2 z -y^2 z -y^2 z -xy^2 -yz^2 -xyz+ x^2 y -xz^2 -x^2 z +y^2 z +xy^2 -yz^2 -xyz+ xyz-x^2 y+xz^2 +y^2 z -xy^2 +yz^2 -xyz`

`= x^2 y -yz^2 +xz^2 -x^2 z +y^2 z - xy^2 .`