请教几道gwd数学，冥思了好久，马上要考了

最近感觉考试的数学难度在增加，请教几道gwd数学，冥思了好久，请各位讨论一下，盼解答

1、If n is a positive integer and r is the remainder when (n-1)(n+l) 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

2、a photographer will arrange 6 people of 6 different heights for photograph by placing them in 2 rows of 3 so that each person in the 1st row is standing in front of someone in the 2nd row. The hegihts of the people within each row must increase from left to right, and each person in the 2nd row must be taller than the person standing in front of him or her. How many such arrangements of the 6 people are possible? (答案是5)

3、DS: A annual rent collected by a corporation from a certain building was x percent more in 1998 than in 1997 and by percent less in 1999 than 1998. Was the annual rent collected by the corporation from the building more in 1999 than in 1997?

(1) x>y

(2) (xy)/100 < x-y

盼解答

3,题目中是否是by y percent less in 1999 than 1998? 如果是

假设:98年是100,则97年为100-X, 99年为100-Y,如果需要1-Y>1-X

所以(1)X>Y充分

看条件2, XY/100<X-Y,可以得到100/Y-100/X>0 同样得到X>Y,所以(2)也充分,因此答案是D,两个都充分

最近感觉考试的数学难度在增加，请教几道gwd数学，冥思了好久，请各位讨论一下，盼解答

1、If n is a positive integer and r is the remainder when (n-1)(n+l) 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

24=2*2*2*3

(1)保证（n-1）（n+1）里有一个能被2整除，一个被4整除

(2)保证（n-1）（n+1）里有一个能被3整除

所以选C

2、
a photographer will arrange 6 people of 6 different heights for
photograph by placing them in 2 rows of 3 so that each person in the
1st row is standing in front of someone in the 2nd row. The hegihts of
the people within each row must increase from left to right, and each
person in the 2nd row must be taller than the person standing in front
of him or her. How many such arrangements of the 6 people are possible?
(答案是5)

编号1-6，1和6固定，中间排剩下4个

3、DS: A annual rent collected by a corporation from a
certain building was x percent more in 1998 than in 1997 and by percent
less in 1999 than 1998. Was the annual rent collected by the
corporation from the building more in 1999 than in 1997?

(1) x>y

(2) (xy)/100 < x-y

设1997是t

1998是t（1+x%）

1999是t(1+x%)(1-y%)

比较t(1+x%)(1-y%)和t的大小

哦,应该是判定XY的符号,所以1不充分.

呵呵

2.按高矮分别为1.2.3.4.5.6号人，

456  256  356  346  246
123  134  124  125  135

1号只能站第一排最左边，6号只能站第二排最右边

3.1997、1998、1999的rent分别是a、b、c

（1+x/100）a=b  (1-y/100)b=c   c-a=(1-y/100)b-b/(1+x/100)=b*[(x/100-y/100-xy/10000)/(1+x/100)]

只要求出x/100-y/100-xy/10000是否大于零即可

只凭条件（1），不能确定C-A是否大于零

只凭条件（2），代入后得出大于零

选B

前后两道明白了

第二题，中间4个怎么排列，能算出5钟方法呢 难道是 p44/2 ?

这三道明白了，谢谢大家

另外两个贴 也请mymengming和lysun和guojiazong看看

http://forum.chasedream.com/dispbbs.asp?boardID=22&ID=328898&page=1

http://forum.chasedream.com/dispbbs.asp?boardID=22&ID=328906&page=1