Two Questions

N =(L - F) / D +1, where N=Number of terms, L=Last term, F=First term, D=Common difference.

N=(73 -1) / 2 + 1

N =37 terms

Sum = (F + L) / 2 x N

Sum = (1 + 73) / 2 x 37

Sum =1,369

Do exactly the same thing with numbers 1 to 231. If you don't make any mistakes, you should get:

Sum =13,456

So you can find the formula.

I checked the results with Excel.

Omi67:

The questioner wants the SUM of these ODD numbers:

{1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27, 29, 31, 33, 35, 37, 39, 41, 43, 45, 47, 49, 51, 53, 55, 57, 59, 61, 63, 65, 67, 69, 71, 73} =37 terms. This is an "arithmetic sequence".

=1,369

At first I had misread the question.
I corrected the answer.

Sorry

Thanks, Omi67.