Shouldn't it be: too?

We have:

- all the sums are cyclic-

Thus: iff (1)

Now: (2)

Let , then (2) is equivalent to: -remember, it's cyclic-

By Cauchy-Schwarz: (3)

Let then :

Thus by (3) we have:

But since by AM-GM inequality -remember -, it follows that which proves (1)

Since and