The sequence a1, a2, a3, ... an of n integers is such that ak=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.
-->n = a(n)
For example:
a1 = 1
a2 = -1
a3 = 3
--->Sum of a1; a2; a3 = 3--->a positive integer-->The sum of odd terms is positive--->Sufficient
(2) a(n) is positive
--->n is odd--->The total number is odd--->The sum of odd terms is positive--->Sufficient
--->The answer is D
(1) n is odd.
-->n = a(n)
For example:
a1 = 1
a2 = -1
a3 = 3
--->Sum of a1; a2; a3 = 3--->a positive integer-->The sum of odd terms is positive--->Sufficient
(2) a(n) is positive
--->n is odd--->The total number is odd--->The sum of odd terms is positive--->Sufficient
--->The answer is D
