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题库 / OG2017Q-DS-287
In the sequence S of numbers, each term after the first two terms is the sum of the two immediately preceding terms. What is the 5th term of S ?
(1) The 6th term of S minus the 4th term equals 5.
(2) The 6th term of S plus the 7th term equals 21.
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.
题目分析：
序列中除前两项为初始项，每一项为其前面相邻两项的和，序列前7项如表所示。
（1）第六项减第四项等于5，又由于第四项加第五项等于第六项，所以第五项就是5。充分。
（2）第6项加第7项等于21，即3a+5b+5a+8b=8a+13b=21，由于不知道a和b的值，也无法确定第5项的值。不充分。
因此，本题答案为（A）。
就是考阅读，考耐心。
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