This editor will not allow me to write the answer for you as usually written, but
1+2+3+...+129 = ∑n from n=1 to n=129.
I do not understand what the A and B are in your question.
Paul M.
tutor
The actual sum is 129*130/2...& the easiest way to see this is to write the series and then reverse the terms and add the 2 series together. You will see that each term is 130 & there are 129 terms.
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05/17/19
Catherine B.
A is the value on top of the sigma and B is the equation of the arithmetic sequence.05/17/19