Thanks a lot mrfantastic!!! =)

But I got a similiar question which I can't solve:

The sum of the first 20 terms of a geometric series is 10 and the sum of the first 30 terms is 91, find the sum of the first 10 terms.

I divided S20 by S10 to eliminate 'a' so as to find 'r':

(r^30 -1) / (r^20 -1) = 91/10

Got to here but I am not sure how to continue. If I expand I will end up with this equation:

10r^30 - 91r^20 + 81 = 0

and the graph of this equation looks kind of weird.

But if I express in this form: 10(r^10)^3 - 91(r^10)^2 +81 = 0

I get r^10 = 9, 1, -9/10

Is there a better way to simplify so I can end up with a simple EQN like the previous question here:

http://www.mathhelpforum.com/math-he...tml#post661852?