数学T paper 2讨论专区

tiewkj

回复 301# walrein_lim88

好，谢谢。

海涵

这题怎么做？
There are 12 towels, two of which are red. If 5 towels are chosen at random, find the probability that at least one is red.

peaceboy

这题怎么做？
There are 12 towels, two of which are red. If 5 towels are chosen at random, find the  ...
海涵 发表于 19-6-2010 01:49 PM

    5C1(1/6)(5/6)^4 + 5C2(1/6)^2(5/6)^3
应该是这样没错 = ='''

Log

回复 303# 海涵
probability= (2C1x10C4)/12C5 + (2C2x10C3)/ 12C5
               = 15/22

Log

回复 303# 海涵
probability= (2C1x10C4)/12C5 + (2C2x10C3)/ 12C5
               = 15/22

peaceboy

回复  海涵
probability= (2C1x10C4)/12C5 + (2C2x10C3)/ 12C5
               = 15/22
Log 发表于 19-6-2010 05:51 PM

    原来.....我想错方向 ....

海涵

多谢帮忙解答。。

海涵

怎么做。。
prove
∫sec x dx  = ln l sec x + tanx l +c

walrein_lim88

怎么做。。
prove
∫sec x dx  = ln l sec x + tanx l +c
海涵 发表于 21-6-2010 07:10 PM

给你下面的CLUE：
    sec x  = sec x ( sec x + tan x)/(sec x + tan x)
             =(sec^2 x + tan x sec x )/(sec x + tan x )
Try RELATED to f'(x)/f(x)

自己在慢慢想吧～～

海涵

回复 310# walrein_lim88


    谢谢哦。。^^

blazex

1)Box A contains 3 black balls and a white ball . Box B has 2 black balls and 3  
   white balls. The probability of choosing box A and box B are 2/3 and 1/3
   respectively. After the box is chosen, a ball is withdrawn from that box. If the  
   ball drawn is black, what is the probability that it was taken from box B?

2)The probability that a girl student chosen at random wears spectacles is 1/12
   and the probabiliy that a student who wears spectacles is a girl is 1/9. The  
   probability that a student selected randomly is not girl and does not wear
   spectacles is 3/4. What is the probability that a student chosn randomly is a   
   girl who wears spectacles?

邪魅

请各位帮帮我！
pelangi2003年版的，page231，地15题，
问题是group data 来滴，他的range差很多，有的只有10，有的有250，
问题：
1.explain y the mean n sd r not neccessarily the best statistical representation for tis distribution.
2.cal. another statistical representation which can be used.

peaceboy

1)Box A contains 3 black balls and a white ball . Box B has 2 black balls and 3  
   white balls. T ...
blazex 发表于 24-7-2010 05:29 PM

    1)Box A contains 3 black balls and a white ball . Box B has 2 black balls and 3  
   white balls. The probability of choosing box A and box B are 2/3 and 1/3
   respectively. After the box is chosen, a ball is withdrawn from that box. If the  
   ball drawn is black, what is the probability that it was taken from box B?

P( Bl black) = p(black n B) / p(black) = (2/5 * 1/3 ) / ( [3/4 * 2/3] + [ 2/5 * 1/3] )
                                                    = 4/１9  

2)The probability that a girl student chosen at random wears spectacles is 1/12
   and the probabiliy that a student who wears spectacles is a girl is 1/9. The  
   probability that a student selected randomly is not girl and does not wear
   spectacles is 3/4. What is the probability that a student chosn randomly is a   
   girl who wears spectacles?

p(s l g ) = 1/12
P(s n g) = p(g) * 1/12  ---- 1

p(g l s ) = 1/9
P( g n s ) = p(s) * 1/9  ------2

1=2 ，
p(s) * 1/9 = p(g) * 1/12
p(s) = 3/4 * p(g) ------3

p(s' n g' ) = 3/4 -----4

we know that ,
p(g' n s) + p (g n s ) = p (s)
sub  1 and 3 into the equa,
p(g' n s) + p(g) * 1/12 = 3/4 * p(g)
p(g' n s) = [3/4 - 1/12]  p(g)
             = 8/12 p(g) -----5

we know that
p(g') = 1- p (g) ----- 6
and
P(g'  n s' ) + P(g' n s ) = p (g') -----7

sub 4, 5 , 6 into 7
3/4 + 8/12 p(g) =  1- p (g)
8/12 p(g) + p (g) = 1/4
20/12 p(g) = 1/4
p(g) = 1/4 * 12/20
       = 12/80


P(s n g) = p(g) * 1/12      
            = 1/80 * 1/12
            = 12/960   
            = 1/80      

乱下，不懂答案有没有错

blazex

回复 314# peaceboy


    第一题的答案是4/19.不是应该选箱子后才拿球吗？为什么不是P(black|B)
    第二题的答案是1/80.
    我也不知道是书的答案是否正确。

peaceboy

回复  peaceboy


    第一题的答案是4/19.不是应该选箱子后才拿球吗？为什么不是P(black|B)
    第二 ...
blazex 发表于 25-7-2010 02:07 PM

    　抱歉少写个１　，　计算方法没错　，　问题问说，现在你已经拿到黑球了，那么这个黑球是从ｂｏｘ　Ｂ拿出来的几率是多少
所以　P( Bl black) 　没错
第二题给我研究下先，有点乱

peaceboy

回复  peaceboy


    第一题的答案是4/19.不是应该选箱子后才拿球吗？为什么不是P(black|B)
    第二 ...
blazex 发表于 25-7-2010 02:07 PM

    抱歉，　２　题做法ｃｏｎｃｅｐｔ都对，只是算错罢了　＝＝

blazex

回复 316# peaceboy


    we know that ,
p(g' n s) + p (g n s ) = p (s)
sub  1 and 3 into the equa,
p(g' n s) + p(g) * 1/12 = 3/4 * p(g)
p(g' n s) = [3/4 - 1/12]  p(g)
             = 8/12 p(g) -----5
会不会是这个部分出现问题？

peaceboy

回复  peaceboy


    we know that ,
p(g' n s) + p (g n s ) = p (s)
sub  1 and 3 into the equa ...
blazex 发表于 25-7-2010 02:21 PM

    　没有，我不小心不把1/4 * 12/20　算成　１／８０罢了

blazex

感激不尽

ultikiller

求解答。22，25，26。
答案
22) (a) 100.25 (b)100.9 (c)10.84
25) 3.87, 2.13
26) (a) 165.1 (b) 2.6