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DS: Sum of the terms in the sequence

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zloebelf
 Post subject: DS: Sum of the terms in the sequence
 Post Posted: Sun Nov 28, 2010 1:35 pm 
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Course Students


Posts: 1
Problem:
Sequence a1,a2,a3,…an of n integers is such that ak = k if k is odd and ak = ak-1if k is even. Is the sum of the terms in the sequence positive?

1) n is odd
2) an is positive

My Interpretation:
k ak
1 1
2 -1
3 3
4 -3
5 5
6 -5
etc

A was the answer I provided. I understand why A is correct, but at least according to my understanding of the sequence, if an is positive, then the sum of the terms is either a positive or zero. Is this sufficient to answer than the sum is positive, even if zero is possible? Isn't zero neither positive nor negative?
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tim
 Post subject: Re: DS: Sum of the terms in the sequence
 Post Posted: Tue Nov 30, 2010 10:10 am 
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ManhattanGMAT Staff


Posts: 2242
Location: Southwest Airlines, seat 21C
To answer your immediate question, 0 is neither positive nor negative. There is a problem with the way you have written the question though, as it appears that the sequence will be as follows:

a1 = 1
a2 = 1
a3 = 3
a4 = 3
a5 = 5
a6 = 5
etc.

If this is true, statement 1 works, but so does statement 2. More importantly though, given the problem as you wrote it, the question is answerable without either statement. This NEVER happens on the GMAT. Can you double check your transcription of the problem?

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Tim Sanders
Manhattan GMAT Instructor
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