Question

How do you find S20 for the geometric series 4 + 12 + 36 + 108 + …?

Answer 1

6973568800

Explanation:

Geometric series with first term `a=4` and common ratio `r=3`.

Sum of geometric series given by

`S_n = (a(1-r^n))/(1-r)`

`S_(20) = (4(1-3^20))/(1-3) = 6973568800`