(x+y)(x^2+y^2)(x-y)(x^4+y^4)=(x+y)(x-y)(x^2+y^2)(x^4+y^4)=(x^2-y^2)(x^2+y^2)(x^4+y^4)=
(x^4-y^4)(x^4+y^4)=x^8-y^8