x^4-xy^3-x^3y-3x^2y+3xy^2+y^4 =（x^4-x^3y）-xy^3+y^4-3x^2y+3xy^2 =x^3(x-y)-y^3(x-y)-3xy(x-y) =x^3-y^3-3xy =(x-y)(x^2+xy+y^2)-3xy =x^2+xy+y^2-3xy =x^2-2xy+y^2 =(x-y)^2 =1