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幼苗
共回答了23个问题采纳率:82.6% 举报
(1)设甲、乙两人摸到的球为红球分别为事件A,事件B,
前四次摸球中甲恰好摸到两次红球为事件C,
则 P(A)=P(B)=
1
3
则 P(C)=P(AA
.
A +A
.
A
.
B A+
.
A
.
B AA)
=
1
3 ×
1
3 ×
2
3 +
1
3 ×
2
3 ×
2
3 ×
1
3 +
2
3 ×
2
3 ×
1
3 ×
1
3 =
14
81
(2)ξ的所有取值分虽为0,1,2
P(ξ=0)=
2
3 ×
1
3 +
2
3 ×
2
3 ×
2
3 =
14
27 ,
P(ξ=1)=
1
3 ×
2
3 +
2
3 ×
2
3 ×
1
3 =
10
27 ,
P(ξ=2)=
1
3 ×
1
3 ×
2
3 =
2
27 ,
P(ξ=3)=
1
3 ×
1
3 ×
1
3 =
1
27 ,
∴ξ的分布列为
ξ 0 1 2 3
P
14
27
10
27
2
27
1
27 ∴ Eξ=0×
14
27 +1×
10
27 +2×
2
27 +3×
1
27 =
17
27 .
1年前
3