Geometric Progression : PS Archive
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# Geometric Progression

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Manager
Joined: 04 Dec 2008
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06 Aug 2009, 22:07
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Question Stats:

67% (01:29) correct 33% (01:45) wrong based on 4 sessions

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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
Manager
Joined: 15 Jun 2009
Posts: 163
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06 Aug 2009, 23:20
IMO C....

Sum 6 terms/sum of 3 terms = 9

a(1-r^6)/(1-r) = 9 { a(1-r^3)/(1-r) }

1 - r^6 = 9 (1 - r^3)
r^6 - 9r^3 + 8 = 0
(r^3 - 8)(r^3 - 1) = 0

r=2 or r=1

r=1 not possible ...as it is increasing G.P. ...so r=2
Re: Geometric Progression   [#permalink] 06 Aug 2009, 23:20
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