There are 12 students including A and B.10 of them are randonly selected one by one.
a)Find the probability that A and B are selected.
b)Find the probability that B is selected right after A.
probability
2012-01-31 1:49 am
回答 (1)
2012-01-31 11:39 am
✔ 最佳答案
a)P(A and B are selected)= 10 students include A and B ways / 10 students ways= (2C2) (10C8) / (12C10)= 15/22b)Method 1 :There are 9 positions for AB since A can be order 1 to 9.P(B is selected right after A)= 9 (10P8) / (12P10)= 3/44
Method 2 :P(B is selected right after A)= P(A and B are selected) * P(A order 1 to 9 when A and B are selected) * P(B is selected right after A when A order 1 to 9)= (15/22) * (9/10) * (1/9)= 3/44
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