How do you find #S_n# for the geometric series #a_1=12#, #a_5=972#, r=-3? Precalculus Series Sums of Geometric Sequences 1 Answer Monzur R. Aug 30, 2017 #S_n = (12(1-(-3)^n))/4# Explanation: The sum to #n# of a geometric series, denoted as #S_n#, is given by the formula #S_n = (a(1-r^n))/(1-r# where #a# is the first term of the series and #r# is the common ratio. #S_n = (12(1-(-3)^n))/(1--3 )= (12(1-(-3)^n))/4# 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 3260 views around the world You can reuse this answer Creative Commons License