GMAT MATH - Arithmetic and Geometric Progression
March 5, 2004
If the ratio of the sum of the first 6 terms of a G.P. to the sum of the first 3 terms of the G.P. is 9, what is the common ratio of the G.P?
| (A) | 3 |
| (B) | 1/3 |
| (C) | 2 |
| (D) | 9 |
| (E) | 1/9 |
Correct Choice (C) Answer (2)
Solution:
The sum of the first n terms of a G.P. is given by  , where ‘a’ is the first term of the G.P., ‘r’ is the common ratio and ‘n’ is the number of terms in the G.P.
Therefore, the sum of the first 6 terms of the G.P will be equal to 
And sum of the first 3 terms of the G.P. will be equal to
The ratio of the sum of the first 6 terms : sum of first 3 terms = 9 : 1
i.e. 
=> 
=> r3 + 1 = 9
=> r3 = 8
=> r = 2
| 