[其他] 離散兩題
題目1:
Throw 7 balls into 5 bins.
What's the probability that at least one of the bins
has precisely 3 balls in it?
想法:
C(n,k) denotes from n choose k
Let A_i be event that i-th bin has precisely 3 balls.
Pr(>=1 bins has precisely 3 balls)
= C(5,1)Pr(A_i) - C(5,2)Pr(A_iA_j) where i!=j
這邊Pr(A_i) = C(7,3) * (1/5)^3 * (4/5)^4 應該沒問題
Pr(A_iA_j) where i != j
= Pr(A_j|A_i)Pr(A_i)
Pr(A_j|A_i) = C(4,3) * (1/4)^3 * (3/4)^1
不知道對不對
求幫釐清觀念
題目二: https://imgur.com/a/4JMnEpM
Monty Hall Problem
想法:
Case 1:選錯後選對
Pr(一開始選錯門): (n-k)/n
Pr(揭開j門後選對): k/(n-j-1)
WAS: win after switching
Pr(WAS1)=[(n-k)/n] * [k/(n-j-1)]
=(k/n) * ((n-k) / (n-j-1))
Case 2:選對後選對
Pr(一開始選對門): k/n
Pr(揭開j門後選對): (k-1)/(n-j-1)
Pr(WAS2)=(k/n) * ((k-1) / (n-j-1))
Sum up 2 cases:
Pr(WAS)=Pr(WAS1)+Pr(WAS2)
=[k(n-1)] / [n(n-j-1)]
= (k/n) * ((n-1) / (n-j-1))
To maximize the ratio between Pr(WAS) and k/n
-> To maximize (n-1) / (n-j-1)
k->1
j->n-2
ratio -> (n-1) / (n-(n-2)-1) = n-1
--
※ 發信站: 批踢踢實業坊(ptt.cc), 來自: 125.230.217.188 (臺灣)
※ 文章網址: https://www.ptt.cc/bbs/Math/M.1603962102.A.E46.html
推
10/29 20:22,
3年前
, 1F
10/29 20:22, 1F
→
10/29 20:22,
3年前
, 2F
10/29 20:22, 2F
→
10/29 20:22,
3年前
, 3F
10/29 20:22, 3F
→
10/29 20:22,
3年前
, 4F
10/29 20:22, 4F
→
10/29 22:52,
3年前
, 5F
10/29 22:52, 5F
→
10/29 22:55,
3年前
, 6F
10/29 22:55, 6F
※ 編輯: LiquidTLO (125.230.217.188 臺灣), 10/29/2020 23:07:12
→
10/29 23:08,
3年前
, 7F
10/29 23:08, 7F
→
10/29 23:10,
3年前
, 8F
10/29 23:10, 8F
→
10/29 23:23,
3年前
, 9F
10/29 23:23, 9F
→
10/29 23:30,
3年前
, 10F
10/29 23:30, 10F
→
10/29 23:30,
3年前
, 11F
10/29 23:30, 11F
→
10/29 23:30,
3年前
, 12F
10/29 23:30, 12F
→
10/29 23:30,
3年前
, 13F
10/29 23:30, 13F
→
10/29 23:47,
3年前
, 14F
10/29 23:47, 14F
※ 編輯: LiquidTLO (125.230.217.188 臺灣), 10/30/2020 00:08:54
※ 編輯: LiquidTLO (125.230.217.188 臺灣), 10/30/2020 00:17:49
→
10/30 00:31,
3年前
, 15F
10/30 00:31, 15F
※ 編輯: LiquidTLO (125.230.217.188 臺灣), 10/30/2020 00:31:52
※ 編輯: LiquidTLO (125.230.217.188 臺灣), 10/30/2020 00:53:04
※ 編輯: LiquidTLO (125.230.217.188 臺灣), 10/30/2020 02:05:11
→
10/30 13:09,
3年前
, 16F
10/30 13:09, 16F
→
10/30 13:11,
3年前
, 17F
10/30 13:11, 17F
→
10/30 13:13,
3年前
, 18F
10/30 13:13, 18F
→
10/30 13:15,
3年前
, 19F
10/30 13:15, 19F
→
10/30 14:24,
3年前
, 20F
10/30 14:24, 20F
→
10/30 14:29,
3年前
, 21F
10/30 14:29, 21F
討論串 (同標題文章)