How to Find Approximate Area Using Sigma Notation Video ? 5min.com

Areas, Riemann Sums, and Definite Integrals Video ? 5min.com

Calculus Videos

(video's 21 to 23!)

YouTube - Calculating a Definite Integral Using Riemann Sums - Part 1

(and part two!).

I think if you watch these videos in order you'll ace Riemann sums & understand everything perfectly.

By the way, the sum you've shown is not a Riemann sum but it is a summation formula.

There is a way to derive it but I suggest to you to first concentrate on understanding a Riemann sum & what it is then come back to tis forum to ask how to derive this formula.