A great starting point for finding summations.
Video Chapters:
Introduction
0:00
Basic Properties
0:07
Summation of i
3:45
Summation of i-squared
6:09
Sum of a Geometric Series
8:10
Summary of Useful Summation Formulas
13:02
Up Next
13:45
Textbook: Rosen, Discrete Mathematics and Its Applications, 7e
Playlist: https://www.youtube.com/playlist?list=PLl-gb0E4MII28GykmtuBXNUNoej-vY5Rz