问题 问答题

一生产线生产的产品成箱包装,每箱的重量是随机的.假设每箱平均重量50千克,标准差为5千克.若用最大载重量为5吨的汽车承运,试利用中心极限定理说明每辆车最多可以装多少箱,才能保障不超载的概率大于0.977(Φ(2)=0.977,其中Φ是标准正态分布函数).

答案

参考答案:

本题是数三考题,设n是所求箱数,又设第i箱的重量是Xi(千克)i=1,2,…n.显然X1,X2…,Xn独立同分布.根据中心极限定理

近似服从正态分布N(50n,25n)

由此可见

,解得n<98.0199

答案最多可以装98箱.

 

单项选择题
阅读理解

阅读理解。

     You can improve your child's hearing memory considerably(在很大程度上). Once his

hearing memory has been greatly improved, he'll be able to use and remember what he listens

to in class. The success of these suggestions depends upon your ability to use your child's

natural desire to get involved in games he finds simple and fun.

     A good way to begin is to read aloud a sentence from a book suitable for your child's

reading age. Then ask your youngster to repeat the sentence back to you correctly. Next,

reread the sentence, leaving out a particular word. See if your child can identify the word

you left out. At the beginning use only simple sentences. Gradually, increase the length of the

sentences. Make sure you don't rush things along too quickly, or your child may become

discouraged and tired of the game.

     Take your child shopping with you often. He's to remember a list of items you want to buy

in the supermarket. First , ask him to remember only a few things. Then, as he shows increased

ability to remember, make the list longer and longer. Praise him often and warmly when he

shows increased ability to remember things. He'll become proud of his 'good' memory and

will happily play the game.

     Encourage your child to learn easy and short poems. As his ability to do this becomes

stronger, encourage him to remember longer poems. Do the same with songs.

1. The suggestions will be successful if____.

A. you find simple and funny games for your child

B. you can stimulate(激发)your child's interest in the activity

C. you force your child to get involved in more practice

D. you improve your own hearing memory first

2. The purpose of asking your child to repeat what you say is ____.

A. to practice his pronunciation

B. to develop his reading skills

C. to help him remember what he hears

D. to play a simple game he may find fun

3. The third paragraph mainly talks about _____.

A. another way to improve your child's hearing memory

B. how you should take you child shopping with you

C. how to remember the items you want to buy

D. the way you help your child do shopping

4. The last sentence 'Do the same with songs' means you should encourage your child to learn

and remember _____.

A. songs in the same way as he does with poems.

B. poems in the same way as he does with songs

C. the poems and songs which are similar in meaning

D. those songs that have the same meaning as poems

5. Who are the supposed readers of this passage?_____

A. Children.

B. Parents.

C. Teachers.

D. Psychologists