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# GMATprep DS: progression

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Manager
Joined: 17 Aug 2009
Posts: 238
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Kudos [?]: 115 [0], given: 25

GMATprep DS: progression [#permalink]  29 Dec 2009, 04:50
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Difficulty:

(N/A)

Question Stats:

0% (00:00) correct 100% (00:08) wrong based on 3 sessions
Need help on this one (attached) -
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GMATPrep AP.doc [69 KiB]

Manager
Joined: 09 May 2009
Posts: 204
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Kudos [?]: 111 [0], given: 13

Re: GMATprep DS: progression [#permalink]  29 Dec 2009, 07:58
IMO D

for k =odd $a_k$ will be $a_1$ =1 ; $a_3$=3 ; $a_5$=5 ; so on

for k=even $a_k$=-$a_(k-1)$ will be $a_2$=-$a_1$=-1; $a_4$=-3 ; $a_6$=-5 :SO ON

SEQUENCE will be

1-1+3-3+5-5+7-7 ( when n is even sum=0 and when n=odd sum=n)

s1) for all n=odd sum=n and hence it is either +1,+3,+5,+7.....SO ON BUT ALL +VE
HENCE SUFF

s2)if $a_n$ is +ve -----> n is odd and hence sum is +ve =n
hence suff
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Manager
Joined: 17 Aug 2009
Posts: 238
Followers: 4

Kudos [?]: 115 [0], given: 25

Re: GMATprep DS: progression [#permalink]  29 Dec 2009, 08:42
Thanks guys
OA is
[Reveal] Spoiler:
D
Re: GMATprep DS: progression   [#permalink] 29 Dec 2009, 08:42
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