x^3(a+1)-xy(x-y)(a-b)+y^3(b+1) =x^3(a+1)-xy(x-y)(a+1)+xy(x-y)(b+1)+y^3(b+1) =x(a+1)(x^2-xy+y^2)+y(b+1)(x^2+y^2-xy) =(x^2+y^2-xy)[(a+1)x+(b+1)y]