问题 填空题
(理科)设ξ是一个离散型随机变量.
(1)若ξ~B(n,p),且E(3ξ+2)=9.2,D(3ξ+2)=12.96,则n、p的值分别为______、______;
(2)若ξ的分布列如表,则Eξ=______.
ξ-101
P
3
4
1-3a2a2
答案

(1)因为ξ~B(n,p),

所以Eξ=np,Dξ=np(1-p)…①

因为E(3ξ+2)=9.2,D(3ξ+2)=12.96,

所以E(3ξ+2)=3Eξ+2=9.2,D(3ξ+2)=9Dξ=12.96,

所以Eξ=2.4,Dξ=1.44…②

所以由①②解得:n=6,p=0.4.

(2)因为

3
4
+(1-3a)+2a2=1,

所以a=

3+
3
4
(舍去)或a=
3-
3
4

所以Eξ=-1×

3
4
+0×(1-3×
3-
3
4
)+2×(
3-
3
4
)
2
=
3-3
3
4

故答案为:6;0.4;

3-3
3
4

写作题

Li Lei是一个13岁的男孩,现就读于光明中学八年级(9)班,他的英语学得很好,他感到很满意,也很高兴。但一年前他的英语却不好。

请根据以上内容写篇短文(不少于50词),把他现在和一年前的感受写出来。(提示:多用“系表”结构)

                                                                              

                                                                              

                                                                              

                                                                              

                                                                              

                                                                              

                                                                              

                                                                              

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