✔ 最佳答案
有的,報酬是黑球個數。
0 個黑球的機率是 (8/12)*(7/11)=14/33
1 個黑球的機率是 (2/12)*(8/11)*2=8/33
2 個黑球的機率是 (2/12)*(1/11)=1/66
所以,黑球個數的期望值是:
0*14/33+1*4/33+2*1/66
=3/11 (個)
2015-02-15 18:11:48 補充:
打錯算式:
0*14/33+1*8/33+2*1/66
(答案無打錯,��������)
2015-02-15 18:22:23 補充:
看錯數字,再來。
0 個黑球的機率是 (8/10)*(7/9)=28/45
1 個黑球的機率是 (2/10)*(8/9)*2=16/45
2 個黑球的機率是 (2/10)*(1/9)=1/45
所以,黑球個數的期望值是:
0*28/45+1*16/45+2*1/45
=18/45
=2/5 (個)
2015-02-15 18:47:40 補充:
若是計算白球個數的期望值,則:
0 個白球的機率是 (2/10)*(1/9)=1/45
1 個白球的機率是 (8/10)*(2/9)*2=16/45
2 個白球的機率是 (8/10)*(7/9)=28/45
所以,白球個數的期望值是:
0*1/45+1*16/45+2*28/45
=72/45
=8/5 (個)