比较a^3-a^2b-3ab与2(a^2)b-6ab^2+b^3的大小（a>b).

a^3-a^2b-3ab^2 -(2ba^2-6ab^2+b^3)
=a^3-a^2b-3ab^2 -2ba^2+6ab^2-b^3
=a^3-3a^2b+3ab^2 -b^3
=a^3-b^3-3a^2b+3ab^2
=(a-b)(a^2+ab+b^2)-3ab(a-b)
=(a-b)(a^2-2ab+b^2)
=(a-b)^3
当a＞b时,a^3-a^2b-3ab^2＞2ba^2-6ab^2+b^3