Recall that Σ is the summation of multiple terms from i->n (sequence and series Pre Calc 12). Here you will learn how to write sums in closed form based on sum identities. You will use these in Riemann Sums (3.4)
How to sum with a sigma function (cr: Organic Chem Tutor)
How to write an arthimetic sequence or series into sigma (cr: Khan Academy)
Closed Form/Summation Formulas (cr: Organic Chem Tutor)
https://www.khanacademy.org/math/ap-calculus-ab/ab-integration-new/ab-6-3/a/review-summation-notation