MATHS AND STATISTICS

2012-04-25 6:09 am
而家學緊PROBABILITY
有一條PP 唔明!!
我有STEPS但唔明!!
Ten seats are arranged in a row for 10 students from 3 different schools.There are 2 students from school A, 4 from school B and 4 from school C. Assume that students from the same school are indistinguishable.

a) Find the number of ways in which these 10 students can take the seats.

b) Find the probability that the 2 students from school A are sitting next to each other.

a) 10!/(2!4!4!) <------點解要除(2!4!4!)???
b) 9!/(4!4!)/3150<---點解要除(4!4!)???


諗咗好奈!!
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THANKS A LOT!!!! :)
更新1:

但點解係除唔係減既???

回答 (1)

2012-04-25 7:43 am
✔ 最佳答案
a)10!/(2!4!4!) <------點解要除(2!4!4!)???這題只是直接用公式,可能你唔熟。題目話 Assume that students from the same school are indistinguishable.
即係話同一間 school 嘅學生係無分別嘅。咁你可以將 A school 的 2個學生當 A , A
B school 的 4 個學生當 B , B , B , B
C school 的 4 個學生當 C , C , C , C咁樣 10個學生本來有 10! 個排列 , 不過 A , A 有2個重複 , 所以要除 2!
B,B,B,B 有4個重複 , 又要除 4!
C,C,C,C 有4個重複 , 都要除 4!所以實際有 10! / (2! 4! 4!) ways。

b)因為 AA 一起坐 , 所以將佢地綁埋當一個人計 ,
咁即係變咗 9個人咁計 , 大同小異。咁樣 9個學生本來有 9! 個排列 , AA 當1個人無重複 , 所以唔洗除
, 或者你可以話佢自己重複自己一個 , 所以要除 1! , (即係等於無除過)
跟住同 a) part 的一樣 ,
B,B,B,B 有4個重複 , 又要除 4!
C,C,C,C 有4個重複 , 都要除 4!所以實際有 9! / (1! 4! 4!) = 9! / (4! 4!) ways。所以 P(E) = 9! / (4! 4!) / All ways , All ways 即 a) part 計了的 10! / (2! 4! 4!) = 3150答案 : P(E) = 9! / (4! 4!) / 3150

2012-04-25 23:59:04 補充:
因為這是公式嘛,

分子係乘大咗 , 分母就除番細。


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