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GMATPrep DS Problems - help!

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by GMAT blows » Tue Aug 21, 2007 10:10 am
Any insights/suggestions will be appreciated. Thnx!

1. The sequence a(1), a(2), ..., a(k) of n integers is such that a(k) = k if k is odd and ak = -a (k-1) if k is even. Is the sum of the terms in the sequence positive?

1) n is odd
2) a(n) is positive
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Source: — Data Sufficiency |
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by lalitgmat » Tue Aug 21, 2007 11:16 am
The approach to such problems would be for a sequence of numbers or if you are brilliant, recall a function that satisfies the condition. I prefer the former one.
a(1) = 1; a(2) = -1; a(3) = 3 ; a(4) = -3 ; a(5) = 5; a(6) = -5
a(-1) = -1; a(-2)= -a(-3)..hence undefined
Also "0" is Neither Even Nor ODD.

Hence , Sum of series is positive if N is ODD. Hence (A) alons is sufficient to answer.

Now, we see than a(n) is positive if n is odd only. Hence if n is odd sum of series is positive also.

Hence EITHER OF THE STMTS ALONE IS SUFFICIENT TO ANS.
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