Find the sum of the first 15 terms of the geometric sequence.

Question

∞
Σ 3n − 1
n = 1

Answer 1

this is just an arithmetic sequence with
a=2
d=3
So, S15 = 15/2 (2*2+14*3)

If you really wanted a geometric sequence, the I assume you meant

Σ3^(n-1)

In that case,
a=1
r=3
so S15 = 1*(3^15-1)/(3-1)