The GMAT Math practice question given below is a sequences and series question on Geometric Progression
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?
The correct choice is (C) and the correct answer is 2
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
+ 1 = 9
=> r = 2
More Questions in Sequences & Series : AP GP