I'm looking for a proof of a validity of the inequation:
(n-1)\sum_{i=1}^{n}x_{i}^{2}\neq 2\sum_{i=1,j=1,j<i}^{n}x_{i}x_{j}
Assumptions:
n\geq 2
\exists (i,j),x_{i}\neq x_{j}
i=1,...,n
j=1,...,n
I have no idea how to prove those non-trivial expressions.