1. newcoder take the mobile phone
Idea: That is, there are a white balls and b black balls in the box. Ask the probability of getting a white ball at the kth time: enumerate the number of options for taking out the ball: A(a, a+b), and then find the kth position The number of programs for the white ball: A(a+b-1,a-1)
Probability is the ratio of the two: a/(a+b)
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