悲催。。。。遛狗。。。没答案，怕误导大家。。。妖儿二肆

lpilikeit

感谢楼主这么多数学狗狗

cielle

谢谢lz，辛苦~~

scauhyl

LZ,加油~~

jizhen1216

谢谢楼主啊

hongy1155

7把钥匙在一个圆形的钥匙盘上，问其中两把车钥匙相邻的概率，7把钥匙里只有2把车钥匙。

6!/7! = 1/7

-- by 会员 sdcar2010 (2010/12/24 23:11:20)

我怎么觉得是6!*2/7!=2/7？

-- by 会员 johnson1120 (2010/12/24 23:24:48)

不会啊

sdcar2010

7把钥匙在一个圆形的钥匙盘上，问其中两把车钥匙相邻的概率，7把钥匙里只有2把车钥匙。

6!/7! = 1/7

-- by 会员 sdcar2010 (2010/12/24 23:11:20)

我怎么觉得是6!*2/7!=2/7？

-- by 会员 johnson1120 (2010/12/24 23:24:48)

不会啊

-- by 会员 hongy1155 (2010/12/27 0:13:11)

Read this:
http://forum.chasedream.com/GMAT_Math/thread-509691-1-1.html

windychen0089

7把钥匙在一个圆形的钥匙盘上，问其中两把车钥匙相邻的概率，7把钥匙里只有2把车钥匙。

6!/7! = 1/7

-- by 会员 sdcar2010 (2010/12/24 23:11:20)

我怎么觉得是6!*2/7!=2/7？

-- by 会员 johnson1120 (2010/12/24 23:24:48)

Yeah, you are right.

-- by 会员 sdcar2010 (2010/12/25 0:03:15)

为什么我觉得是P55*2/P66=5!*2/6!, 其它５把进行排列，然后两把车钥匙排列?

sdcar2010

If n different people are sitting around a table, the possibility of 2 persons sitting next to each other is 2/(n-1).

1) Total possible arrangements of n people on a table = n! / n = (n-1)!
2) Total possible arrangements of n-1 people (assuming those two sitting together would be treated as a single person) = 2* (n-1)! / (n-1) = 2* (n-2)!  There is a 2 in the equation because the two persons are not identical.

The possibility of 2 persons sitting next to each other = 2* (n-2)! / (n-1)! = 2/(n-1)