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# GMATprep DS: progression

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Manager
Joined: 17 Aug 2009
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29 Dec 2009, 05:50
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0% (00:00) correct 100% (00:08) wrong based on 3 sessions

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Manager
Joined: 09 May 2009
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29 Dec 2009, 08:58
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: 235
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Kudos [?]: 182 [0], given: 25

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29 Dec 2009, 09:42
Thanks guys
OA is
[Reveal] Spoiler:
D
Re: GMATprep DS: progression   [#permalink] 29 Dec 2009, 09:42
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