How do you find #S_n# for the arithmetic series given #a_1=76#, #a_n=176# and n=21? Precalculus Series Sums of Arithmetic Sequences 1 Answer Ratnaker Mehta Jun 23, 2017 #S_n=(n/2)(a_1+a_n) rArr S_21=21/2(a_1+a_21)=21/2(76+176)# #=2646.# Answer link Related questions How do I find the sum of an arithmetic sequence? What is the formula for the sum of an arithmetic sequence? What are common mistakes students make when finding the sum of an arithmetic sequence? How do I find the sum of an arithmetic sequence on a calculator? How do I find the partial sum of an arithmetic sequence? How do I find the partial sum of an arithmetic sequence on a TI-84? What is meant by the sum of an arithmetic sequence? How do I find the sum of the arithmetic sequence 3, 5, 7, 9, ..., 21? What is the sum of the arithmetic sequence 22, 13, 4? How do you find the sum of the first 25 terms of the sequence: 7,19,31,43...? See all questions in Sums of Arithmetic Sequences Impact of this question 1525 views around the world You can reuse this answer Creative Commons License