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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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Is the sum of the terms in the sequence 【positive】
不需要知道具体的数值...
做数学的时候老是被鬼打墙！！！！我瞎了！！！
归因：分析错误。数列项数不可能小于1
positive。。。
我这一拳下去。。。一定会把我自己打哭把
发现他们的规律 1 + (-1) + 3 + (-3) + 5 + (-5) + ..... ，所以两个都是充分
题目是Is the sum .......positive
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