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GMAT Math Practice - Geometric Progression

The GMAT Math practice question given below is a sequences and series question on Geometric Progression

Question 5

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?
  1. 3
  2. 1/3
  3. 2
  4. 9
  5. 1/9
The correct choice is (C) and the correct answer is 2.

Explanatory Answer

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



More Questions in Sequences & Series : AP GP

1. Sum of all 3 digit numbers leaving remainder .. 2. Number of 3 digit numbers leaving remainder ..
3. 9th term of an arithmetic sequence 4. Sum of 1st 15 terms given 4th plus 12th term
5. Given sum of geometric progression; find r. 6. Sum of 2 sets of even numbers.
7. DS : Finding x, a missing term in an AP. 8. Finding number of terms of an AP.
9. DS : Finding the 6th term of an AP. 10.
 


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