Summation of Products of n Numbers taken m at a time with Repetitions/Examples/Order 2

Theorem
Let $a, b \in \Z$ be integers such that $b \ge a$.


 * $\displaystyle \sum_{i \mathop = a}^b \sum_{j \mathop = a}^i x_i x_j = \dfrac 1 2 \left({\left({\sum_{i \mathop = a}^b x_i}\right)^2 + \left({\sum_{i \mathop = a}^b {x_i}^2}\right)}\right)$