题目
题型:不详难度:来源:
2 |
3 |
1 |
2 |
(I)求甲、乙两班各有1人获奖的概率;
(II)求该校获奖人数ξ的分布列与期望.
答案
Bi表示乙班有i人获奖,i=0,1,2.
P(Ak)=
C | k2 |
2 |
3 |
1 |
3 |
P(Bi)=
C | i2 |
1 |
2 |
1 |
2 |
据此算得P(A0)=
1 |
9 |
4 |
9 |
4 |
9 |
P(B0)=
1 |
4 |
1 |
2 |
1 |
4 |
甲、乙两班各有1人获奖的概率为P(A1B1) =P(A1)P(B1) =
4 |
9 |
1 |
2 |
2 |
9 |
(II)ξ的所有可能值为0,1,2,3,4,且
P(ξ=0)=
1 |
9 |
1 |
4 |
1 |
36 |
P(ξ=1)=
1 |
9 |
1 |
2 |
4 |
9 |
1 |
4 |
1 |
6 |
P( A0 •B2)+P(A1B1)+P(A2B0)=
13 |
36 |
P(ξ=3)=
4 |
9 |
1 |
4 |
4 |
9 |
1 |
2 |
1 |
3 |
P(ξ=4)=
4 |
9 |
1 |
4 |
1 |
9 |
综上知ξ的分布列