How do you find #S_n# for the geometric series #a_1=72#, r=1/3, n=7? Precalculus Series Sums of Geometric Sequences 1 Answer Binayaka C. Nov 19, 2017 # S_n ~~ 107.95 # Explanation: #a_1=72, r=1/3 ,n=7 , S_n=?# Summation formula: #S_n= a_1*(1-r^n)/(1-r)# #= 72*{1-(1/3)^7)/(1-1/3)~~ 107.95 (2dp)# # S_n ~~ 107.95 (2dp)# [Ans] 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 1339 views around the world You can reuse this answer Creative Commons License