Question

How many terms are there in a geometric series if the first term is 7, the common ratio is 2, and the sum of the series is 105?

Hint: (image attached below) r ≠ 1, where a1 is the first term and r is the common ratio.

OPTIONS
A) n = 3
B) n = 4
C) n = 5
D) n = 6

Answer 1

the answer would be B. N=4

Answer 2

105 = 7. (1 - 2^n) / 1-2

105*-1 = 7 (1 - 2^n)

1 - 2^n = -105/7 = - 15

2^n = 16

n = 4 answer