a^3 b^3 c^3-3abc =[( a b)^3-3a^2b-3ab^2] c^3-3abc =[(a b)^3 c^3]-(3a^2b 3ab^2 3abc) =(a b c)[(a b)^2-(a b)c c^2]-3ab(a b c) =(a b c)(a^2 b^2 2ab-ac-bc c^2)-3ab(a b c) =(a b c)(a^2 b^2 c^2-ab-ac-bc) 用到二个公式：a^3 b^3=(a b)(a^2-ab b^2) (a b)^3=a^3 b^3 3a^2b 3ab^2
