作者twosix (好好珍惜...)
看板Statistics
标题Re: [问题]一题期望值的问题
时间Sun Sep 14 20:50:14 2008
※ 引述《penatnl (penatnl)》之铭言：
: Suppose n balls are distributed at random into r boxes.
: Let Xi=1 if box i is empty and =0 otherwise.
: (1)compute E(Xi)
: (2)For i=/=j(i不等於j) compute E(XiXj)
E(Xi)=P(BOX i is empty)
C (n+r-2)取n r-1
= ----------------= ------------
C (n+r-1)取n n+r-1
E(XiXj)=P(BOX i & BOX j are empty)
C (n+r-3)取n
=--------------
C (n+r-1)取n
有错请订正~
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