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

[Guest]

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

[Guest]

where did you get this problem from?

[Guest]

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.

[Guest]

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

[Guest]

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

How do you get A2 = -3?

[TMart82]

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?

[Guest]

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 ..

[RonPurewal]

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?