Question

PLEASE HELP! Infinite geometric series again. I am very, VERY confused and need some help finding out the answer. Attachment below.

Answer 1

First, a geometric series is a series of the form a + ar + ar^2 + ...

Can you figure out what a and r are?

Answer 2

Let me try. Hang on

Answer 3

Not really, because the first number is 3-1, and that's one of the reasons I'm so confused.

Answer 4

The first number in the sequence is 3, not 3-1. Thus, a must be 3.

Answer 5

Do you agree?

Answer 6

I'm going to agree, because you seem like you know what you're doing.

Answer 7

Is "a" the sum? The sum would be 3?

Answer 8

LOL, ok. So, "a" is the "a" in "a + ar + ar^2 + ...". That is, it is just the first number in the sequence.

Answer 9

Since we are given that the first number is 3, that means -- as a pattern matching problem -- "a" = 3 in the geometric series.

Answer 10

Now, let's identify what r must be. We know a=3, and we know the second number in the sequence is -1. So, this is a simple problem: ar = 3r = -1, solve for r.

Answer 11

Will you solve for r, and the final step after that is to use the geometric sum formula, which depends on "a" and "r".

Answer 12

Is this the same formula I'd use for the finite geometric series?