Question

what is the use of sigma ?

Answer 1

sigma tells us about summation
it means of adding. 3 times or sometimes number is defined on it

Answer 2

Sigma notation provides a way to compactly and precisely express any sum, that is, a sequence of things that are all to be added together. Although it can appear scary if you’ve never seen it before, it’s actually not very difficult. 

We would read this as “the sum, as k goes from a to b, of f(k).” In plain English, what this means is that we take every integer value between a and b (inclusive) and substitute each one for k into f(k). This results in a bunch of values which we add up.

Hope This Helps.