Question

The first term of a geometric series is -3, the common ratio is 6, and the sum of the series is -4,665. Using a table of values, how many terms are in this geometric series?

Answer 1

Option A. 5

Explanation:

From the question given above, the following data were obtained:

First term (a) = –3

Common ratio (r) = 6

Sum of series (Sₙ) = –4665

Number of term (n) =?

The number of terms in the series can be obtained as follow:

Sₙ = a[rⁿ – 1] / r – 1

–4665 = –3[6ⁿ – 1] / 6 – 1

–4665 = –3[6ⁿ – 1] / 5

Cross multiply

–4665 × 5 = –3[6ⁿ – 1]

–23325 = –3[6ⁿ – 1]

Divide both side by –3

–23325 / –3 = 6ⁿ – 1

7775 = 6ⁿ – 1

Collect like terms

7775 + 1 = 6ⁿ

7776 = 6ⁿ

Express 7776 in index form with 6 as the base

6⁵ = 6ⁿ

n = 5

Thus, the number of terms in the geometric series is 5.