show that: (a-b)^2 (a+b) + ab (a+b) = a ^ 3 + b^3 (a – b)^2 = a^2 - 2ab + b^2 ab(a+ b) = a^2b + ab^2 (a^2 – 2ab + b^2)(a + b) + a^2b + ab^2 = a^3 -2a^2b + ab^2+ a^2b -2ab^2 + b^3 + a^2b + ab^2 = a^3 – 2a^2b + 2a^2b – 2ab^2 + 2ab^2 + b^3 = a^3 + b^3 proved