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数学T paper 2讨论专区
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刚才在做练习时。。关于Chapter6--Probability..
不知道是我的答案错。。还是书本的答案错。。
所以希望有人可以帮帮我。。
题目是。。
Given that universal set S = all integers from 1 to 100,
A= set of numbers which are complete squares and B= set of multiples of 3,
find
(a) n(A U B) 【Ans :41 】
(b) n(A ∩ B) 【Ans :2 】
可是。。我的答案却是
(a) 40
(b) 3
所以,希望获得高人的指点。。谢谢。。
[ 本帖最后由 猪头鱼 于 3-5-2009 08:19 PM 编辑 ] |
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发表于 23-3-2009 01:18 AM
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回复 1# bell_25 的帖子
显然书本答案错了 |
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楼主 |
发表于 23-3-2009 01:30 AM
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哦。。谢谢你们的帮忙 。。 |
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发表于 23-3-2009 04:22 PM
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楼主 |
发表于 12-4-2009 11:31 PM
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这里有一题关于probability的问题我不会。。
可以帮个忙吗?
题目是:
A ball contains 7 identical red balls, 4 identical white balls and 9 identical black balls. 3 balls are taken out from the box one after another without replacement. What is the probability that at least one ball is white?
[ ans: 29/57 ]
谢谢。。。 |
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发表于 12-4-2009 11:47 PM
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回复 6# bell_25 的帖子
提示
P(W) = 1 - P(W')
W = at least 1 white ball |
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发表于 12-4-2009 11:51 PM
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原帖由 bell_25 于 12-4-2009 11:31 PM 发表
这里有一题关于probability的问题我不会。。
可以帮个忙吗?
题目是:
A ball contains 7 identical red balls, 4 identical white balls and 9 identical black balls. 3 balls are taken out from the box one ...
P(X>=1) = 1-P(X=0)
= 1-......
=29/57 |
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楼主 |
发表于 12-4-2009 11:54 PM
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回复 7# iamverynoob 的帖子
哦。。我知道这个。。
我知道怎样做了。。
可是。。我想问。。
P(W')的算法是不是拿 16C3 / 20C3 呢??
可是题目有说明balls are taken out from the box one after another without replacement....这样做ok吗?? |
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楼主 |
发表于 13-4-2009 12:13 AM
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回复 8# 笨蛋一个 的帖子
不好意思。。我不是很明白这个。。
这个不是probability distribution的方法吗??
还是我误解了。。方便解释吗? |
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发表于 13-4-2009 05:32 PM
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原帖由 bell_25 于 12-4-2009 11:54 PM 发表
哦。。我知道这个。。
我知道怎样做了。。
可是。。我想问。。
P(W')的算法是不是拿 16C3 / 20C3 呢??
可是题目有说明balls are taken out from the box one after another without replacement....这样做ok吗 ...
是ok的~
另一種方法是
P(W) = 1 - (16/20) X (15/19) X (14/18)
= 29/57 |
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发表于 13-4-2009 05:34 PM
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原帖由 bell_25 于 13-4-2009 12:13 AM 发表
不好意思。。我不是很明白这个。。
这个不是probability distribution的方法吗??
还是我误解了。。方便解释吗?
應該不是probability distribution~
他的方法和iamverynoob的方法是一樣的~ |
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发表于 13-4-2009 07:45 PM
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对,我的方法和iamverynoob一样
发这个贴时才知道原来有人比我快了 |
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楼主 |
发表于 13-4-2009 10:07 PM
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回复 11# Ivanlsy 的帖子
嗯。。明白了。。谢谢你的解答。。 |
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楼主 |
发表于 13-4-2009 10:17 PM
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回复 13# 笨蛋一个 的帖子
哈哈。。不好意思。。
原来我真的误解了。。
可是我还是有点blur。。
题目说明contains 7 identical red balls, 4 identical white balls and 9 identical black balls.。。。那么为什么不可以写成
P(W) = 1 - (16/20) X (15/19) X (14/18) X 3!/ 3! (for 3 identical red ball)
或
P(W) = 1 - (16/20) X (15/19) X (14/18) X 3!/ 3! (for 3 identical black ball)
或
P(W) = 1 - (16/20) X (15/19) X (14/18) X 3!/ 2! (for 2 identical red ball and 1 black ball)
或
P(W) = 1 - (16/20) X (15/19) X (14/18) X 3!/ 2! (for 2 identical black ball and 1 red ball)
呢??
不好意思哦。。你们明白我写什么吗?? |
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发表于 14-4-2009 11:09 AM
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楼主 |
发表于 15-4-2009 02:13 AM
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回复 16# 笨蛋一个 的帖子
嗯。。谢谢你的解答。。
我想我明白了。。哈哈。。。
因为我一直以为要把各种不同颜色的球当成不同的group。。
原来是我自己搞错了。。 |
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楼主 |
发表于 3-5-2009 12:42 PM
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也是一题和probability有关的题目。。
请指教。。题目是:
A day is either sunny or rainy. The probability that it is sunny for two consecutive days is 0.8. The probability that a rainy day is followed by a sunny day is 0.4. The probability that the weather forecast expects 1st October to be sunny is 0.75.
(a) What is the probability that 2nd October is sunny? What is the probability that 3rd October is also sunny?
(b) If 1st October is sunny, what is the probability that 3rd October is rainy?
(c) If 3rd October is rainy, what is the probability that Ist October was sunny?
【Ans: (a) 0.7, 0.68 ; (b) 0.28 ; (c) 0.65625 】
其实我不是很明白这题目。。
我和朋友尝试做了(乱乱撞的)。。可是答案都和书本里的不一样。。
所以只好麻烦你们了。。谢谢。。 |
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发表于 3-5-2009 05:01 PM
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原帖由 bell_25 于 3-5-2009 12:42 PM 发表
A day is either sunny or rainy. The probability that it is sunny for two consecutive days is 0.8. The probability that a rainy day is followed by a sunny day is 0.4. The probability that the weather forecast expects 1st October to be sunny is 0.75.
(a) What is the probability that 2nd October is sunny? What is the probability that 3rd October is also sunny?
(b) If 1st October is sunny, what is the probability that 3rd October is rainy?
(c) If 3rd October is rainy, what is the probability that Ist October was sunny?
(a) P(2nd October is sunny) = (0.75 × 0.8) + (0.25 × 0.4)
= 0.7
P(3rd October is also sunny) = 0.7 × 0.8 + (0.75)(1 - 0.8)(0.4) + (0.25)(1 - 0.4)(0.4)
= 0.68 (这题的问题问得不明确)
(b) P(3rd October is rainy) = [0.8 × (1 - 0.8)] + [(1 - 0.8) × (1 - 0.4)]
= 0.28
(c) 好像怪怪的~你查一下问题抄得对不对?
[ 本帖最后由 Ivanlsy 于 3-5-2009 05:07 PM 编辑 ] |
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楼主 |
发表于 4-5-2009 12:17 AM
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回复 19# Ivanlsy 的帖子
谢谢你。。哈哈。。原来是这样。。
嗯。。我查过了。。问题是这样的。。我没抄错喔。。 |
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发表于 4-5-2009 04:32 PM
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