HELP: I will award medal and become a fan.

What is the sum of the geometric sequence 8, –16, 32 … if there are 15 terms?

I know I have to solve it with Sn=a1-a1r^n
--------
1-r
n=15
a1=8
r=?
How do I get r???

Question

HELP: I will award medal and become a fan. 

What is the sum of the geometric sequence 8, –16, 32 … if there are 15 terms?

I know I have to solve it with Sn=a1-a1r^n
                                                      --------
                                                         1-r
n=15
a1=8
r=?
How do I get r???

primeralph · Answer

r = any term divided by the time before it.

anonymous · Answer

What do you mean?

primeralph · Answer

Divide the second term by the first term.

primeralph · Answer

Or the third by the second....same thing.

anonymous · Answer

Start with 8 and 16, and find r
Then formula for the sum of a geometric series to find the sum up to the 15th term

anonymous · Answer

And that will be r?

primeralph · Answer

Yeah.

anonymous · Answer

Will that work for any equation I have to solve like this?

primeralph · Answer

If it's a geometric series.

anonymous · Answer

Okay. Thanks so much.