SOLUTION: A G.P has the second term as 9 and the fourth term is as 81.find the some of the first four term?

Algebra ->  Sequences-and-series -> SOLUTION: A G.P has the second term as 9 and the fourth term is as 81.find the some of the first four term?       Log On


   

Question 1191585: A G.P has the second term as 9 and the fourth term is as 81.find the some of the first four term?
Answer by greenestamps(13200) About Me  (Show Source):
You can put this solution on YOUR website!


The 4th term 81 is the 3nd term 9, multiplied by the common ratio twice:

81=9r%5E2
9=r%5E2
r=3 or r=-3

There are two possible answers to the problem.

(1) r=3; the first four terms are 3, 9, 27, 81; sum 120
(2) r=-3; the first four terms are -3,9,-27, 81; sum 60