[分房问题]有n个人,每个人都等可能地被分配到N(n≤N)间房中的每一间中,试求下列各事件的概率:(1)指定的n间房中各有一人;(2)恰有n间房中各有一人;(3)指定的某间房中恰有r(r≤n)个人;(4)第1间房,第2间房,…,第N间房中分别有r1,r2,…,rn个人(r1+r2+…+rN=n,D≤ri≤n).分析:这n个 There are n individuals, each person is equally likely to be allocated to each room in the N (n ≤ N) room, try the probability of the following events: (1) designated n rooms There is one person each; (2) there are exactly one room in each; (3) there is a r(r≤n)person in a designated room; (4)the first room, the second room,... In the Nth room, there are r1, r2, ..., rn individuals (r1+r2+...+rN=n, D≤ri≤n). Analysis: n