GMAT考题：OG2017Q-DS-219

GMAT考题

What is the sum of the first four terms of sequence S ?

(1) After the first two terms of S, the value of each term of S is equal to the average (arithmetic mean) of the last two preceding terms.

(2) The average (arithmetic mean) of the first three terms of S is 10.

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.

毕出

题目分析：

本题求序列S前4项的和。

（1）前两项之后，每一项为其前两项的平均值，但由于前两项的值不知道，所以无法求得前4项的和。

（2）条件2给出了前3项的平均值，所以前三项的和为30，但无法得知第4项，也无法求前4项的和。

综合条件1和条件2，由于不知道前两项的值，也无法计算第4项的值，前4项的和无法计算。

因此，本题答案为（E），两个条件结合起来也不能解题，必须提供新数据。

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