求助两题，GWD31的02数学

maxwell5858

请问像一下这类题目如何分析，我总是对这类题目很苦恼

There are 8 magazines lying on a table; 4 are fashion magazines and the other 4 are sports magazines.  If 3 magazines are to be selected at random from the 8 magazines, what is the probability that at least one of the fashion magazines will be selected?

 

A.     1/2

B.     2/3

C.     32/35

D.     11/12

E.      13/14

Answer: E

There are 8 books on a shelf, of which 2 are paperbacks and 6 are hardbacks.  How many possible selections of 4 books from this self include at least one paperback?

 

A.     40

B.     45

C.     50

D.     55

E.      60

 

Answer: D

wyzt001

至少的这种题目尽量从排除性入手

1.[C(3,8)-C(3,4)]/C(3,8) 就是从所有可能性中排除都是sports magazines的可能性C(3,4)

2.C(1,2)*C(3,6)+C(2,2)*C(2,6)