∑ is called Sigma, which is a summation symbol and represents the cumulative term.

n

∑ai=a 1+a2+a3+...+ An.

i= 1

The following i= 1.

"I" means that I is a variable in the general term formula, which increases with the increase of the number of terms 1.

"1" means the change from i= 1,

The "n" above means adding to i=n,