问题 解答题

一个房间有3扇同样的窗子,其中只有一扇窗子是打开的.有一只鸟自开着的窗子飞入这个房间,它只能从开着的窗子飞出去.鸟在房子里一次又一次地向着窗户飞去,试图飞出房间.鸟飞向各扇窗子是随机的.

(1)假定鸟是没有记忆的,若这只鸟恰好在第x次试飞时飞出了房间,求试飞次数x的分布列;

(2)假定这只鸟是有记忆的,它飞向任一窗子的尝试不多于一次,若这只鸟恰好在第y次试飞时飞出了房间,求试飞次数y的分布列.

答案

(1)由题设条件知,试飞次数x遵从几何分布,∴试飞次数x的分布列如下:

x12n
P
1
3
2
3
×
1
3
(
2
3
)n-1×
1
3
(2)由题设条件知P(y=1)=
1
3
P(y=2)=
1
3
P(y=3)=
1
3

∴试飞次数y的分布列如下:

y123
P
1
3
1
3
1
3

阅读理解

阅读理解.

     For an American, or a foreigner here, any chance to have a social life is welcome. But

what happened last night made me think I may be better off having no life while I'm in

Baghdad.

     Friday evening,l was on my way with an American journalist friend to his welcome party.

When we got to a checkpoint,we were told to get out of the car to be searched,which is

normal procedure. We waited under a tent,as our car was examined on the other side

of a wall. We couldn't see what was happening but we figured we would hit the road soon.

     But the search was taking longer than expected, probably because of a car bomb

threat that had been reported earlier. Then we noticed the officers put their helmets(头盔)

on. Never a good sign.

     Our Iraqi driver then noticed that our other Iraqi driver and guards who were in a

second car were standing facing a wall. We had gotten separated from them at the

checkpoint. We told two American officers that they were with us. They whispered

with each other and then walked away.

     A few minutes later, an officer walked toward us and asked us to follow him. He

said to me, "If you make any moves,l will take you down hard. "

     That's when l knew we were actually thought to be doing something terrible. We

were told to sit in a row on the sidewalk, facing the street. I noticed across the street

were our other driver and guards. They were also in a row, but in a kneeling position

with their hands on their heads.

     I was shocked. The only explanation could be that our guards had been at the

shooting range in the morning, so the police dog may have picked something up from

that. I also wondered why our Iraqi fellows on the other side of the street were being

treated worse than us when it was our car that was under suspicion(怀疑) .

      After the end of the hour long suffering, we were allowed to leave. No longer in

the mood for the party,we all went home.

1. The author and his people were going to the party____.

A. in two cars

B. in one car

C. under the protection of police officers

D. for the purpose of exploding a car

2. The author became aware the situation was worse than expected when__________.

A. they arrived at the checkpoint

B. the officers put their helmets on

C. an officer shouted at him

D. some people were asked to kneel down

3. Paragraphs 6 and 7 are mainly written to_________.

A. analyze the reason for the accident

B. describe the author's terrible feelings

C. state the importance to obey police

D. show how differently the officers treated them

4. The author indicated that in Iraq ,__________.

A. he would think twice before going to social activities

B. it was particularly dangerous for foreigners

C. foreigners were treating local people very cruelly

D. American officers could do whatever they wanted

问答题 简答题