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The sequence A1, A2, A3....An of n....

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Guest
 Post subject: The sequence A1, A2, A3....An of n....
 Post Posted: Sat Sep 13, 2008 9:18 pm 
DS:

The sequence A1, A2, A3....An of n integers is such that Ak=k if k is odd and Ak= -Ak-1 if k=even. Is the sum of the terms in the sequence positive?
1) n is odd
2) An is positive


Any takers?
ans: D
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Guest
 Post subject:
 Post Posted: Sun Sep 14, 2008 3:17 pm 
where did you get this problem from?
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Guest
 Post subject:
 Post Posted: Sun Sep 14, 2008 4:34 pm 
GMAT prep. it was from my first full length quant section.

just to clarify the question a bit, the numbers (and k) next to "A" should be subscript.
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Guest
 Post subject:
 Post Posted: Sun Sep 14, 2008 9:08 pm 
The sequence should be 1,-3,3,-5,5,-6,6... (based on the information given)

1) n is odd

Sum of the terms will be positive, since the a1, ie, 1, would remain at the end

2) an will be positive only if it is odd.

So D
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Guest
 Post subject:
 Post Posted: Thu Oct 09, 2008 3:52 pm 
I dont understand the Ak=-Ak-1 bit... is (k-1) a subscript, or just k?

How do you get A2 = -3?
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TMart82
 Post subject: how do you know first term = 1
 Post Posted: Wed Oct 22, 2008 8:08 pm 
How do you know the first term in the sequence is 1? Why couldn't it be 2? If the first term (a1) in the sequence is 2 then it makes no sense, since there is no a0 term. Am I off base on this?
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Guest
 Post subject:
 Post Posted: Fri Oct 31, 2008 4:03 am 
original statement:
A1, A2, A3....An
k is odd: Ak=k
k is even: Ak= -Ak-1

therefore,
A1 (k = 1, the subscript) = 1
A2 (k = 2, the subscript) = -2 - 1 = -3
and so forth ..
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RonPurewal
 Post subject:
 Post Posted: Wed Nov 12, 2008 8:55 am 
Offline
ManhattanGMAT Staff


Posts: 7146
Anonymous wrote:
I dont understand the Ak=-Ak-1 bit... is (k-1) a subscript, or just k?


yeah, the original poster should clear this up; it's ambiguous.

if (k - 1) is a subscript, then a sub 2 = -(a sub 1), which means that (a sub 2) should be -1.
if that's actually supposed to be -(a sub k) - 1, then (a sub 2) should be -1 - 1, or -2.

i'm with the posters here: i don't really smell how (a sub 2) could be -3. could whoever originally wrote that please elaborate?
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