How do you find Sn for the geometric series a1=12, a5=972, r=-3? Precalculus Series Sums of Geometric Sequences 1 Answer Monzur R. Aug 30, 2017 Sn=12(1−(−3)n)4 Explanation: The sum to n of a geometric series, denoted as Sn, is given by the formula Sn=a(1−rn)1−r where a is the first term of the series and r is the common ratio. Sn=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 32, 38? 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 3435 views around the world You can reuse this answer Creative Commons License