Mathematics Q

2007-06-26 9:34 pm
A fair coin is tossed five times. What is the probability that it lands head-up at least twice?

回答 (3)

2007-06-26 10:05 pm
✔ 最佳答案
P(head at least twice)
= 1 - P(no head) - P(head only show once)
= 1 - (1/2)^5 - [(1/2)^5]*5

(1/2)^5 --> all 5 are tail, no head
(1/2)^5 * 5 --> only 1 head, others are tail, ie. HTTTT, THTTT, TTHTT, TTTHT & TTTTH

= 1 - 1/32 - 5/32
= 26/32
= 13/16
2007-06-28 1:46 am
For only one head,

Favoriable outcome are:HTTTT,THTTT,TTHTT,TTTHT,TTTTH

         1  1  1  1  1
P(only a head)=(--- x ----- x ---- x ----- x ----- )x 5
         2  2  2  2  2

         5
       = ----
        32

For no head,

Favoriable outcome is:TTTTT

       1  1  1  1  1
P(no head)= ---- x ----- x ----- x ---- x -----
       2  2  2  2  2

       1
        = -----
         32

P(at least two head)=1-P(only a head)-P(no head)

            5    1
         =1- ------ - -------
           32  32 

          26
         = -----
          32

          13
         = -----
          16
            ==
2007-06-26 9:47 pm
1/2 x 1/2 =1/4
參考: my knowledge


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