刚做了一套GWD，有两个数学还是没思路。不好意思，一时搜不到前人的帖子，还请各位帮忙看看，月底考试了，急啊。
4,For a nonnegative integer n, if the remainder is 1 when 2n is divided by 3, then which of the following must be true?
I. n is greater than zero.
II. 3n = (-3)n
III. √2n is an integer.
A. I only
B. II only
C. I and II
D. I and III
Answer: E
Q15:
If n is a positive integer and r is the remainder when (n – 1)(n + 1) is divided by 24, what is the value of r?
(1) 2 is not a factor of n.
(2) 3 is not a factor of n.
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.
Answer: C
第一题：
N 2^n 除以3的余数
0 1 1
1 2 2
2 4 1
3 8 2
4 16 1
可看出：根据条件n是偶数，且n可以等于0，所以选E。
第2题：
条件（1）说明n是奇数，（n-1）和(n+1)则是连续的偶数，但除以24（因子为3和3个2）余数不确定
条件（2）说明（n-1）和(n+1)其中有一个可被3整除且同时为奇数或同时为偶数，但除以24（因子为3和3个2）余数也不确定
条件（1）＋ 条件（2）可得（n-1）*(n+1)可被3*2*2整除，同时最小的符合条件的n是1,5，7，11……n>=5时(n-1)或(n+1)中必有一个是4的倍数所以除以24（因子为3和3个2）余数可确定为0，而当n=1时，(n-1)(n+1)=0,所以除以24余数为0，故可确定R=0, 所以选C
lz 送上NO1.,解题思路，仅供参考。
题目已知：a nonnegative integer n so ，n 为0 或者正整数
又 2^n /3 余数为 1 ， 带进去数字 得 n＝0，2，4，6，8。。。。。 余数都是 1
1）n>0 错
n都是偶数，所以 2，3 正确
多谢两位！
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