谁能用两种方法证明a^3+b^3+c^3大于等于3abc（a,b,c属于正实数）

(1)a^3+b^3+c^3-3abc
=(a+b)^3+c^3-3ba^2-3ab^2-3abc
=(a+b+c)[(a+b)^2-(a+b)c+c^2-3ab(a+b+c)
=(a+b+c)(a^2+2ab+b^2-ac-bc+c^2-3ab)
=(a+b+c)(a^2+b^2+c^2-ab-ac-bc)
=1/2(a+b+c)[(a-b)^2+(b-c)^2+(c-a)^2]
a+b+c>0,[(a-b)^2+(b-c)^2+(c-a)^2]>=0
1/2(a+b+c)[(a-b)^2+(b-c)^2+(c-a)^2]>=0
(a"=a^1/3)
(2)(反证法)假设a^3+b^3+c^3