Geometric Series

The Sum of a Geometric Series

The formula is

Where
r is the common ratio
a is first term
n is the number of terms
S_n is the sum of the first n terms

Worked Example 1

Find to the nearest integer the sum of the first 11 terms of the geometric series with 1^st term 5 and 2^nd term 6

First term (a) = 5, common ratio (r) = 6/5 = 1.2

S_n = 5(1.2¹¹ - 1) / (1.2 - 1) = 161