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