✔ 最佳答案
第一題已答:
https://hk.knowledge.yahoo.com/question/question?qid=7014100400153
2014-11-06 19:36:14 補充:
圖片參考:
https://s.yimg.com/lo/api/res/1.2/ZQit8r6YM3UMER1bF6V.OQ--/YXBwaWQ9dHdhbnN3ZXJzO3E9ODU-/http://i.imgur.com/ZH9B85L.gif
1.
圖片參考:
https://s.yimg.com/rk/HA00430218/o/1831022021.png
Pr( 一月一日是雨天 | 一月三日是晴天 )
= Pr( 一月一日是雨天 及 一月三日是晴天 ) / Pr( 一月三日是晴天 )
= (0.2) [(0.3)(0.75) + (0.7)(0.3)] /
{ (0.8) [(0.75)(0.75) + (0.25)(0.3)] + (0.2) [(0.3)(0.75) + (0.7)(0.3)] }
= 0.087 / ( 0.51 + 0.087 )
= 29/199
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2.
題目應該是指袋子中有 4 個黃球, 2 個綠球 和 1 個紅球。
由於每次抽出的球都會放回袋中, 因此, Pr(抽紅球) =
4/(4+2+1) = 4/7。
A 首先抽球, A 勝出的概率是
1/7 + (6/7)(6/7)(1/7) + (6/7)(6/7)(6/7)(6/7)(1/7) + (6/7)(6/7)(6/7)(6/7)(6/7)(6/7)(1/7) + ...
= (1/7) / [1 - (6/7)(6/7)]
= (1 * 7) / [49 - (6)(6)]
= 7/13
圖片參考:
https://s.yimg.com/rk/HA00430218/o/545489186.jpg
2014-11-06 21:34:51 補充:
a + ar + ar² + ar³ + ... = a/(1 - r) 是等比數列無限項之和。
2014-11-06 21:36:28 補充:
應該是中六才學的。
考慮
S = a + ar + ar² + ar³ + ...
S = a + r(a + ar + ar² + ...)
S = a + rS
S - rS = a
S(1 - r) = a
S = a/(1 - r)
2014-11-06 21:36:53 補充:
對於本題
a = 1/7
r = (6/7)(6/7) = 36/49
2014-11-07 19:10:18 補充:
問:
是無止境地加落去?所以要用你所說的 a/(1 - r)
答:
對。