ξ的分布列为
（II）ξ的数学希望为Eξ=0×
| s |
| s+t |
| st |
| (s+t)2 |
| st2 |
| (s+t)3 |
| stn−1 |
| (s+t)n−1 |
| tn |
| (s+t)n |
| t |
| s+t |
| st2 |
| (s+t)2 |
| (n−2)stn−1 |
| (s+t)n−1 |
| (n−1)stn |
| (s+t)n−1 |
| ntn+1 |
| (s+t)n+1 |
（1）-（2）得Eξ=
| t |
| s |
| tn |
| s(s+t)n−1 |
| (n−1)tn |
| (s+t)n−1 |
| ntn |
| (s+t)n |
| s |
| s+t |
| st |
| (s+t)2 |
| st2 |
| (s+t)3 |
| stn−1 |
| (s+t)n−1 |
| tn |
| (s+t)n |
| t |
| s+t |
| st2 |
| (s+t)2 |
| (n−2)stn−1 |
| (s+t)n−1 |
| (n−1)stn |
| (s+t)n−1 |
| ntn+1 |
| (s+t)n+1 |
| t |
| s |
| tn |
| s(s+t)n−1 |
| (n−1)tn |
| (s+t)n−1 |
| ntn |
| (s+t)n |