问题 解答题

已知5个乒乓球,其中3个新的,2个旧的,每次取1个,不放回的取两次,  

求:(1)第一次取到新球的概率.

(2)第二次取到新球的概率.

(3)在第一次取到新球的条件下第二次取到新球的概率.

答案

(1);(2);(3)

题目分析:(1)此问为古典概型的概率,总的基本事件的个数为5个,第一次取到新球的基本事件包含3个,所以;

(2)第二次取到新球包含两种情况,第一次取到新球,或是第一次没有取到新球;

(3)此问为条件概率,根据公式

设第i次取到新球为事件,第j次取到旧球为事件.(i,j=1,2)

(1)                  4分

(2) 第二次取到新球为C事件,  

    8分

(3)                      12分

阅读理解

The earth is changing. Tomorrow it will be different. All the living things on the earth are changing. Even people are changing, and tomorrow you will be different.

There are many kinds of change, and many different things than cause change. People need to realize the changes, to learn to see them, and to learn what causes change.

Human beings must solve many problems so that they can continue to live successfully. War is one of these problems. Disease and natural disaster are problems. Having enough food and energy for the growing number of people in the world is a problem. Most problems can be solved, but some are impossible to solve. The way in which people solve problems affects how long and how well they can live.

As people solve their problems, they change things, and changes often cause new problems for people and for other living things on the earth. Sometimes the new problems seem even worse than the old ones. Human beings will probably never stop having problems to solve. Can you use your imagination to describe a world that has no problems at all ?

小题1:The reason why everything is changing is that______.

A.people want them to be changing

B.there’re many different things that cause them to change

C.the living things are changing

D.the world is too small小题2:People need to solve problems because______.

A.there will be more and more new problems if they don’t solve the old ones.

B.the problems are difficult to solve

C.they have to go on living

D.there are so many things that are changing小题3:New problems appear ______.

A.because old problems aren’t solved.

B.because people don’t solve them

C.because some people are foolish

D.when people are solving old ones小题4:Which of the following if wrong according to the passage ?

A.Disaster and war are two of the problems people have to solve.

B.People need to solve all the problems at the same time.

C.The way people solve problems has much to do with their life.

D.People can’t solve all the problem at one time.小题5:The underlined word “disaster” in the passage probably means “______” in Chinese.

A.灾难

B.资源

C.条件

D.环境

选择题