Sequences & Summation Notation

Go to this link and learn how to  use the Summation Calculator. I have already entered your problem into the calculator and see the answer at the bottom (10,600):

https://www.mathsisfun.com/numbers/sigma-calculator.html

This is an arithmetic series   (AS)

First term  =  2(1)  +  5   = 7

Last term    = 2(100) + 5 =  205

Sum  of  an  AS   =   [ first term + last term ]  * number of terms   / 2   ....so we  have....

[  7  +  205 ]  (100 )  /  2   =

[ 212] * 50   =

10600