gmat prep 1

[jainrahul1985]

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

(1) n is odd.
(2) an is positive.

OA D

[Maciek]

Hi!

we know that ak = k if k is odd.
(1)
If n is odd, then an = n and an is positive.
Since statements (1) and (2) mean the same, we should eliminate answers A, B and C.

The formula for sum of the terms in the sequence:
S = 0.5n(a1 + an)
if k = 1, then a1 = 1
n is odd, so an = n
S = 0.5n(1 + n)
We know that n is positive, so S is also positive.
We should eliminate answer E and choose answer D.

Hope it helps!
Best,
Maciek