问题 解答题

一个口袋内装有大小相等编号为a1,a2,a3的3个白球和1个黑球b.

(1)从中摸出2个球,求摸出2个白球的概率;

(2)从中连续取两次,每次取一球后放回,求取出的两球恰好有1个黑球的概率.

答案

(1)由题意知本题是一个古典概型,

试验发生包含的事件是从4个球中摸出两个球,共有C42=6种结果,

满足条件的事件是摸出2个白球,共有3种结果,

根据古典概型概率公式得到P=

1
2

(2)由题意知本题是一个古典概型,

试验发生包含的事件是4×4=16,

满足条件的事件是确定一个黑球和一个白球,共有3+3=6种结果,

根据古典概型概率公式得到P=

3
8

判断题
单项选择题

Robert Moody, 52, is an experienced police officer. Much of his work involves dealing with (16) and gang (团伙)problems in the schools of his community. Knowing that many kids often (17) trouble, he decided to do something about it. So in 1991 he began to invite small groups of kids to go fishing with him on his day (18) .

Those fun trips had a(n) (19) impact. A chance encounter in 2000 proved that. One day, (20) working security at a school basketball game, Moody noticed two young guys (21) . He sensed trouble between them. (22) one of them headed toward Moody and gave him a hug. "I (23) you. You took me (24) when I was in fifth grade. That was one of the (25) days of my life. "

Deeply touched by the boy’ s word, Moody decided to create a foundation (基金会)that (26) teenagers to the basics of fishing in camping programs. "As a policeman, I saw (27) there was violence, drugs were always behind it. They have a damaging (28) on the kids," says Moody.

By turning kids on to fishing, he (29) to present an alternative way of life, "When you’ re sitting there waiting for a (30) ," he says, "you can’t help but talk to each other, and such (31) can be pretty deep. "

"Talking about drugs helped prepare me for the peer(同龄人)pressures in high school," says Michelle, 17, who (32) the first program. "And I was able to help my little brother (33) drugs. "

Moody faces (34) in three years, when he hopes to run the foundation full-time. "I’ m living a happy life and I have a responsibility to my (35) to give back," Moody says. "If I teach a kid to

fish today, he can teach his brother to fish tomorrow. \

27().

A. where

B. unless

C. as

D. whether