问题 解答题

袋中装着标有数字1,2,3,4,5的小球各2个,从袋中任取3个小球,按3个小球上最大数字的9倍计分,每个小球被取出的可能性都相等。用ξ表示取出的3个小球上的最大数字,求:

(1)取出的3个小球上的数字互不相同的概率;

(2)随机变量ξ的概率分布和数学期望;

(3)计分介于20分到40分之间的概率。

答案

解:(1)“一次取出的3个小球上的数字互不相同”的事件记为A,

(2)由题意ξ有可能的取值为:2,3,4,5,

所以随机变量ξ的概率分布为

因此ξ的数学期望为Eξ=

(3)“一次取球所得计分介于20分到40分之间”的事件记为C,

则 P(C)=P(“ξ=3”或“ξ=4”)=P(“ξ=3”)+P(“ξ=4”)=

阅读理解

Another small animal that tigers eat in Ranthambhore (a wildlife park in northern India) is the porcupine (豪猪).These animals are spread widely across India in open areas and grasslands; they often live in the earth and between rocks.They are good diggers, and in Ranthambhore are nocturnal (夜出的); they only come out at night.They weigh 12 to 16 kilograms and reach the length of 80 to 90 centimetres.They live usually on vegetables and fruit and seem to have a good sense of smell.Porcupines defend themselves when attacked by moving backwards with their quills upright, often making a strange noise.

Several times, I have found the remains of quills where a tiger has killed and eaten a porcupine, but I have never actually seen the kill.In spite of the quills, experienced tigers will try to kill clearly by biting the porcupine’s head, its weakest spot.But the quills are very sharp and can give painful injuries to young tigers.If the quills have gone in very deeply or if they are stuck in the tiger’s paw, it will try to pull them out.Often, it will be successful, and the wound will get better.But, if the quills have gone in very deeply or if they are stuck in the tiger’s neck or mouth, where they cannot be reached, the wound will often turn bad.If this happens, the tiger is in pain and is less able to hunt animals.It has to look for easier prey and so may turn to cows on the edges of the forest.From this, it is a small step to becoming a man-eater.

小题1: A porcupine is a small animal which _____.

A.likes to stay on vegetables and fruit

B.eats nothing but vegetables and fruit

C.prefers to stay on vegetables and fruit rather than anywhere else

D.has vegetables and fruit as its main food小题2: The word “quill” here means ____.

A.teeth of a porcupine

B.feathers on a porcupine

C.long, sharp needle-like parts on a porcupine

D.paws of a porcupine小题3:According to this passage, what causes a tiger to become a man-eater?

A.When there are no porcupine for a tiger to catch.

B.When it turns to cows on the edges of the forest.

C.When a tiger wasn’t hurt seriously.

D.When it is difficult for a tiger to catch other animals.小题4:What is the proper title for this passage?

A.A Tiger and Its Food.

B.How Do Tiger Kill Porcupines?

C.What Can Cause a Tiger to Become a Man-eater?

D.A Porcupine.

判断题