Question

a geometric series where the first term is -12, the last term is -972, and each term after the first is triple the previous term​

Answer 1

the geometric series is a(n) = -12(3)^(n-1)

Explanation:

"Triple" denotes multiplication by 3. Thus, the common factor here is 3.

The general formula for a geometric series is a(n) = a(1)(r)^(n-1), where a(1) is the first term, r is the common ratio.

Here, we have a(n)= (-12)(3)^(n-1) = -972.

We need to solve this for n, which represents the last term.

The first step towards solving for n is to divide both sides by -12:

3^(n-1) = 81

To solve for n-1, rewrite 81 as 3^4. Then we have:

3^(n-1) = 3^4, implying that (n-1) = 4 and that n = 5.

Then we know that it is the 5th term that equals -972.

In summary, the geometric series is a(n) = -12(3)^(n-1).