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问题 解答题

一个袋中有20个大小相同的小球，其中记上0号的有10个，记上n号的有n个（n=1，2，3，4）．现从袋中任取一球，用ξ表示所取球的标号．

（1）求ξ的分布列的数学期望和方差；

（2）若η=aξ+b，E（η）=2，D（η）=44，试求a、b的值．

答案

（1）由题设知ξ=0，1，2，3，4，

P（ξ=0）=

10
20
=
1
2
，

P（ξ=1）=

1
20
，

P（ξ=2）=

2
20
=
1
10
，

P（ξ=3）=

3
20
，

P（ξ=4）=

4
20
=
1
5
，

∴ξ的分布列为：

ξ01234
P
1
2
1
20
1
10
3
20
1
5
…（3分）

∴Eξ=0×

1
2
+1×
1
20
+2×
1
10
+3×
3
20
+4×
1
5
=1.5．…（4分）

Dξ=(0-1.5)2×

1
2
+(1-1.5)2×
1
20
+(2-1.5)2×
1
10
+(3-1.5)2×
3
20
+(4-1.5)2×
1
5
=2.75．…（6分）

（2）由η=a2Dξ，得a2×2.75=44，即a=±4，…（8分）

又Eη=aEξ+b，

∴当a=4时，由2=4×1.5+b，得b=-4；

当a=-4时，由2=-4×1.5+b，得b=8．

∴

a=4
b=-4
或
a=-4
b=8
即为所求．…（12分）

阅读理解

阅读理解。

     Many boys and girls love to watch TV. They spend many hours a day in front of the TV sets. But

many parents let their children watch TV only in special time.     TV shows aren't like books or movies.

There are many types of shows, such  as  sitcoms , soap  operas, sports shows , fashion  shows, etc. A  

child  can  learn  good  things and bad things from them. Some shows help children to understand the

news all over the world. Children don't have to go to the zoo to see animals. Boys and girls can see

sitcoms, sports shows and game shows at home. Some shows teach children how to cook or how to use

tools(工具) .     Many boys and girls think it is fun to watch TV ，but it is also fun to read books, friends. to play games or to visit.

1. Many boys and girls spend_____.

A. four hours on TV shows on Sunday

B. six hours on TV shows every day

C. many hours a day in front of the TV sets

D. two hours in front of the TV sets on weekend

2. Many parents_____. 

A. don't let their children watch TV

B. ask their children to watch TV

C. let their children watch TV in special time

D. ask their children to go outside on weekend  

3. From TV , boys and girls can_____.

A. learn good things and bad things    

B. learn nothing    

C. learn  good things    

D. learn how to use a computer  

4. Boys and girls like to watch TV because_____.  

A. it's fun    

B. they have nothing to do    

C. it's exciting    

D. it's boring

5. What does the writer think of the TV shows?  

A. They are good for children.    

B. They are bad for children.    

C. Some of them are good for children. Others are not.

D. They are very exciting.
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多项选择题

人文素养的内涵可分为如下层次（）。

A.掌握特定的人文社会科学基础知识的程度；

B.在日常生活中运用这些基础知识的能力；

C.以这些知识处理人与自身、他人、社会和自然的关系中形成的特定价值观，及建立在这种价值观基础上的行为模式和趋向；

D.向他人传播这种知识和价值观的效果
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