Try

来源: 于 2015-02-09 16:08:45 [博客] [旧帖] [给我悄悄话] 本文已被阅读：次

The idea is first apply the inequality x1^2 + x2^2 + ... + xn^2 >= (X1+x2+...+xn)^2/n, then use the trick for Prob 8 again.