How do you find #S_n# for the geometric series #a_1=5#, r=3, n=12? Precalculus Series Sums of Geometric Sequences 1 Answer Sahar Mulla ❤ Jan 22, 2018 #S_n = a_1 (r^n-1)/(r-1)# hence plugging in the values, #S_n = 5 ((3^12-1)/(3-1))# #S_n = 5/2 (3^12-1)# Answer link Related questions What is a sample problem about finding the sum of a geometric sequence? What is the formula for the sum of a geometric sequence? What is a sample problem about finding the sum of a geometric sequence? How do I find the sum of the geometric sequence #3/2#, #3/8#? What is the sum of the geometric sequence 3, 15, 75? What is the sum of the geometric sequence 8, 16, 32? How do I find the sum of the geometric series 8 + 4 + 2 + 1? How do you find the sum of the following infinite geometric series, if it exists. 2 + 1.5 +... How do you find the sum of the first 5 terms of the geometric series: 4+ 16 + 64…? How do you find S20 for the geometric series 4 + 12 + 36 + 108 + …? See all questions in Sums of Geometric Sequences Impact of this question 3265 views around the world You can reuse this answer Creative Commons License