问题 解答题

设15个同类型的零件中有2个次品,每次任取1个,共取3次,并且每次取出后不再放回.若以X表示取出次品的个数.

(I)求X的分布列;

(II)求X的数学期望E(X)和方差D(X).

答案

(I)X的取值为0,1,2,

P(X=0)=

C02
C313
C315
=
22
35

P(X=1)=

C12
C213
C315
=
12
35

P(X=2)=

C22
C113
C315
=
1
35

故X的分布列为:

X012
P
22
35
12
35
1
35
(II)E(X)=1×
12
35
+2×
1
35
=
2
5

D(X)=

22
35
×(0-
2
5
2+
12
35
×(1-
2
5
2+
1
35
×(2-
2
5
2=
52
175

阅读理解

In the year after my mother left us, my father was 42 and he knew that he wasn’t going to make it to 43. He wrote a letter to me and hoped that something in it would help me for the rest of my life.

Since the day I was 12 and first read his letter, some of his words have lived in my heart. One part always stands out. “Right now, you are pretending to be a time killer. But I know that one day, you will do something great that will set you among the very best.” Knowing that my dad believed in me gave me permission to believe in myself. “You will do something great.” He didn’t know what I would be, and neither did I, but at times in my life when I’ve felt proud of myself, I remember his words and wish he were here so I could ask, “Is this what you were talking about , Dad ? Should I keep going?”

A long way from 12 now, I realize he would have been proud when I made any progress. Lately, though, I’ve come to believe he would want me to move on to what comes next: to be proud of , and believe in somebody else. It’s time to start writing my own letters to my children. Our children look to us with the same unanswered question we had. Our kids don’t hold back because they’re afraid to fail. They’re only afraid of failing us. They don’t worry about being disappointed. Their fear ---- as mine was until my father’s letter---- is of being a disappointment.

Give your children permission to succeed. They’re waiting for you to believe in them. I always knew my parents loved me. But trust me: That belief will be more complete , that love will be more real, and their belief in themselves will be greater if you write the words on their hearts :“Don’t worry; you’ll do something great .”Not having that blessing from their parents may be the only thing holding them back.

小题1:We learn from the text that the author__________.

A.lost his father when he was young

B.worked hard before he read his father’s letter

C.asked his father’s permission to believe in himself

D.knew exactly what great thing his father wanted him to do小题2:What does the author tell us in the 3rd paragraph?           

A.Children need their parents’ letters.

B.Children are afraid to be disappointed.

C.His children’s fear of failure held them back.

D.His father’s letter removed his fear of failing his parents.小题3:Which of the following is true of the author?

A.He got no access to success.

B.He wrote back to his father at 12.

C.He was sure his parents loved him.

D.He once asked his father about the letter.小题4:The main purpose of the text is to        .

A.describe children’s thinking

B.answer some questions children have

C.stress the importance of communication

D.advise parents to encourage their children

不定项选择