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题库 / Prep2007E1-DS-79
If the terms of a sequence are t1, t2, t3, . . . , tn, what is the value of n ?
(1) The sum of the n terms is 3,124.
(2) The average (arithmetic mean) of the n terms is 4.
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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...还是要写出来自己算下
【引用】
1)t1+t2+t3+...+tn=3124,t1,t2,t3的特点未知，不能求出n
2)只知道平均数，不知道总数也不能求出n
1)+2) 4n=3124 n=781
所以无论tn的表达式怎么变化，n可以求出来
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