求教GWD数学3-16

sycathyjc · 发表于 2011-4-6 10:45:22

Q16:
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.

如题，请教怎么做啊？

sdcar2010 · 发表于 2011-4-6 11:15:58

1) 2 is not a factor of n. So n is odd. n=1, r =0; n=3, r=8. Insufficient
2) 3 is not a factor of n. So either (n-1) or (n+1) has a factor of 3. n=1, r=0; n=4, r= 15. Insufficient.

1) and 2). So n is odd and n is not a factor of 3. If so, 1) both n-1 and n+1 are even; 2) one of them has a factor of 4; 3) one of them has a factor of 3. So n=2*4*3*m = 24*m. r=0. Sufficient.

CCCCCCCCCCCCCc

pandaforever · 发表于 2011-4-6 11:24:33

这个是不是选E呀？？

这样做的:

第一个条件说n不能被2整除，假设n=3或n=5，得到的余数不一样，所以不能确定r;

第二个条件说n不能被3整除，假设n=2或n=4，得到的余数不一样，所以也不能确定r。

pandaforever · 发表于 2011-4-6 11:24:52

等一下，看看两个一起的情况哈

pandaforever · 发表于 2011-4-6 11:25:58

不好意思呀，选C，一起的时候就可以了，余数都是0

sycathyjc · 发表于 2011-4-6 18:51:45

thax~~~~

sycathyjc · 发表于 2011-4-6 18:52:21

thax~~~~

求教GWD数学3-16

所属分类: GMAT考试

正在浏览此版块的会员 ()

浏览过的版块