If S1 = 0.7 and S2 = 2.1 in a geometric series, determine the sum of the first 12 terms in the series. Be sure to show all of your work.

Answer 1

a = 0.7
a+ar = 2.1
so,
0.7(1+r) = 2.1
1+r = 3
r = 2

So, now you know that

S12 = a(1-r^12)/(1-r)

Just plug in your values.