Using summation operator

Hi!

I have very basic questions and problems with summation operator. Please see the attached worksheet for examples. First question, I'm not just able to understand how the high lighted sum (13) is calculated?

Second example is shown also in the worksheet. How would you use the summation operator so that you could prepare worksheet template and define the n in the beginning of the calculation and then the formula adapts with the amount of n? I mean so that you don't have to change the formula by hands every time the n changes?

Thanks in advance and sorry for the beginner level questions 😣

Br, Harri