Forget conventional ways of solving math questions. In DS, Variable approach is the easiest and quickest way to find the answer without actually solving the problem. Remember equal number of variables and equations ensures a solution.
If sequence a_1,a_2,a_3,.........,a_n of n integers is such that ak=k if k is odd and a_k=-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
In the original condition there are multiple variables and since we need just as many equations, E is likely the answer. Using both 1) & 2) together, 1) =2) therefore D is likely the answer.
Assuming n=odd=3, a_1=1, a_2=-a_1=-1, a_3=3 and the last term is always left out. Therefore the the sum is positive. The answer is yes, and the condition is sufficient. Therefore the answer is D.
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