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
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Geometric Progression
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joyseychow
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Sum of GP= a * (r^n - 1) / (r - 1)
here the ratio of sum of 6 terms by sum of 3 terms
=> (r^6 - 1) / (r^3 -1 ) = 9
=> r^6 - 9r^3 + 8 = 0
=> (r^3 - 1)(r^3 - 8) = 0
So IMO Ans = 2 i.e. C
here the ratio of sum of 6 terms by sum of 3 terms
=> (r^6 - 1) / (r^3 -1 ) = 9
=> r^6 - 9r^3 + 8 = 0
=> (r^3 - 1)(r^3 - 8) = 0
So IMO Ans = 2 i.e. C
