a^3+b^3
=a^3+a^2b-a^2b+b^3+ab^2-ab^2
=(a^3-a^2b+ab^2)+(b^3-ab^2+a^2b)
=a(a^2-ab+b^2)+b(b^2-ab+a^2)
=(a+b)(a^2-ab+b^2)