Question

The first term of a geometric series is 3, and the sum of the first term and the second term is 15. What is the sum of the first six terms?

A: 1,023
B: 3,906
C: 4,096
D: 11,718

Answer 1

Let a = first term
r = common ratio

a = 6
a + ar = 15
a(1+r) = 15

r = 3/2

So sum of first 6 terms where n=6 is given by:

\(S_n = \frac{a(r^n - 1)}{r-1}\)

Nwo put in the values, you'll get the answer. :)

Understood? :)

Answer 2

But isn't the first term 3?