错了，应该反一反(a+b)^3=(a+b)^2*（a+b) =(a^2+2ab+b^2)(a+b) =a^3+2a^2b+ab^2+a^2b+2ab^2+b^3 =a^3+3a^2b+3ab^2+b^3a^3-3a^2b＋3ab^2-b^3＝(a-b)^3
