2. 外语
  3. If S is the sum of the first n positive integers, what is the value of n ? (1) S220

If S is the sum of the first n positive integers, what is the value of n ? (1) S220

admin2022-10-18  26
问题 If S is the sum of the first n positive integers, what is the value of n ?
(1) S<20
(2) S2 >220
选项 A、Statement (1) ALONE is sufficient, but statement (2) alone is not sufficient.
B、Statement (2) ALONE is sufficient, but statement (1) alone is not sufficient.
C、BOTH statements TOGETHER are sufficient, but NEITHER statement ALONE is sufficient.
D、EACH statement ALONE is sufficient.
E、Statements (1) and (2) TOGETHER are NOT sufficient.
答案C
解析 (1)    Given that S < 20, n could be 5 since = 15 < 20. However, n could also be 4 since；
NOT sufficient.
(2)    Given that S2 > 220, n could be 5 since = 15 and 152 = 225 > 220.
However, n could also be 6 since = 21 and 212 = 441 > 220;
NOT sufficient.
Given (1) and (2) together, it is clear from the following table that n = 5.

The correct answer is C;
both statements together are sufficient.
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