x^4+y^4+(x+y)^4 =(x^2+y^2)^2-2x^2y^2+(x+y)^4 =[(x+y)^2-2xy]^2-2x^2y^2+(x+y)^4 =[(x+y)^2]^2-4xy(x+y)^2+4x^2y^2-2x^2y^2+(x+y)^4 =2(x+y)^4-4xy(x+y)^2+2x^2y^2 =2[(x+y)^4-2xy(x+y)^2+(xy)^2] =2[(x+y)^2-xy]^2 =2(x^2+xy+y^2)^2