问题 解答题
甲、乙两人各射击一次,击中目标的概率分别是
2
3
3
4
,假设两人每次射击是否击中目标相互之间没有影响.
(Ⅰ)求甲射击5次,有两次未击中目标的概率;
(Ⅱ)求两人各射击4次,甲恰好击中目标2次,且乙恰好击中目标3次的概率.
答案

(I)设“甲射击5次,有两次未击中目标”为事件A,

P(A)=

C25
(
2
3
)3•(
1
3
)2=
80
243

答:甲射击5次,有两次未击中目标的概率为

80
243
.(6分)

(Ⅱ)设“两人各射击4次,甲恰好击中目标2次,且乙恰好击中目标3次”为事件B,

P(B)=

C24
(
2
3
)2•(
1
3
)2
C34
(
3
4
)3
1
4
=
1
8

答:两人各射击4次,甲恰好击中目标2次,且乙恰好击中目标3次的概率为

1
8
.(13分)

单项选择题
阅读理解

The public commonly associates steroid use with big-time athletics. But the drugs may be even more of a menace to teenagers. The synthetic hormones can stunt a young person’s growth by prematurely closing the ends of the long bones in the skeleton. That means a 1.7 metre, 15-year-old high school student who uses steroids “might get bigger but won’t get any taller”, according to a US doctor.

Nearly seven percent of boys in the US try steroids before the end of high school, according to a 1988 study by professors Charles Yesalis and William Buckley, of Penn State University. The estimate comes from a poll of 3,400 seniors in 46 public and private schools across the US.

“Abusers of Steroids did it to improve their appearance and to excel at sports,” Professor Yesalis said. “Parents, teachers and coaches make boys believe that to be an ideal male you need to have these. Then they say: ‘you can’t play games to have fun; you play games to win.’”

“Such attitudes put many high school athletes at high risk of becoming abusers of steroids,” said Mike Gimbel, director of Baltimore County Office of substance Abuse. “For these athletes, the pressure to perform is incredible,” he said. “It was inevitable that it would seep down to high school level.”

小题1:The word “big-time” in Paragraph 1 most nearly means “________”.

A.long time

B.good time

C.top rank

D.modern小题2:Why are steroids even more of a threat to teenagers?

A.Because they make teenagers grow too tall.

B.Because they make teenagers get too big.

C.Because they prevent teenagers from getting taller and taller.

D.Because they give teenagers too much pressure.小题3:Which of the following statements is implied but not mentioned?

A.School athletes have too much pressure.

B.Steroids can help people have lots of muscles.

C.The pressure chiefly comes from parents, teachers and coaches.

D.Girls are not interested in steroids.小题4:Which of the following is a good title for this passage?

A.Drugs and Sports

B.The Pressure to School Boys are Too Much

C.School Boys are Interested in Drugs

D.School Athletes Risk Damage from Drugs