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题库 / Prep2007E1-DS-125
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.
Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient.
Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient.
BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient.
EACH statement ALONE is sufficient.
Statements (1) and (2) TOGETHER are NOT sufficient.
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ooo, 通过举例带入，发现他们的规律 1 + (-1) + 3 + (-3) + 5 + (-5) + ..... ，所以两个都是充分
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