问题 选择题
甲、乙两个篮球运动员互不影响地在同一位置投球,命中率分别为
1
2
与p,且乙投球2次均未命中的概率为
1
16
.若甲、乙两人各投球2次,两人共命中2次的概率是(  )
A.
3
16
B.
11
32
C.
9
64
D.
21
64
答案

设“甲投球一次命中”为事件A,“乙投球一次命中”为事件B.

由题意得 (1-P(B))2=(1-p)2=116

解得 p=

3
4
5
4
(舍去),

∴乙投球的命中率为

3
4

甲、乙两人各投球2次,共命中2次有三种情况:

甲、乙两人各中一次;甲中两次,乙两次均不中;甲两次均不中,乙中2次.

概率分别为

1
2
×
1
4
+
1
2
×
3
4
+
1
2
×
1
2
×
1
4
×
1
4
+
1
2
×
1
2
×
3
4
×
3
4
=
11
32

∴甲、乙两人各投两次共命中2次的概率为

11
32

故选B.

选择题
阅读理解

阅读理解。

     As they migrate (迁移), butterflies and moths choose the winds they want to fly with, and they change

their body positions if they start floating in the wrong direction. This new finding suggests that insects may

employ some of the same methods that birds use for traveling long distances. Scientists have long thought

that insects were simply at the mercy of the wind.

     Fascinating as their skills of flight are, migrating behavior has been difficult to study in insects because

many long distant trips happen thousands of feet above ground. Only recently have scientists developed

technologies that can detect such little creatures at such great heights.

     To their surprise, though, the insects weren't passive travelers on the winds. In autumn, for example,

most light winds blew from the east, but the insects somehow sought out ones that carried them south and

they positioned themselves to navigate directly to their wintering homes.

     Even in the spring, when most winds flowed northward, the insects didn't always go with the flow. If

breezes weren't blowing in the exact direction they wanted to go, the insects changed their body positions

to compensate. Many migrating birds do the same thing.

     The study also found, butterflies and moths actively flew within the air streams that pushed them along.

By adding flight speeds to wind speeds, the scientists calculated that butterflies and moths can travel as fast

as 100 kilometers an hour. The findings may have real-world applications. With climate warming,migrating

insects are growing in number. Knowing how and when these pests move could help when farmers decide

when to spray their crops.

1. What's the main idea of the text? [ ]

A. Insects migrate with the seasons.

B. Wind helps insects greatly in migrating.

C. Windsurfing insects have real direction.

D. Scientists have trouble in observing insects.

2. Scientists originally thought that _____. [ ]

A. insects always waited for their favourable winds

B. insects chose the winds they wanted to ride

C. insects were just blown about by the wind

D. insects positioned themselves in the winds

3. It is not easy to study the migrating behavior of the insects because _____. [ ]

A. the little creatures can fly very fast

B. they have no regular migrating courses

C. the wind's direction is hard to foresee

D. their flight is long and high above ground

4. We can learn from the text that _____. [ ]

A insects fly in the way birds do

B. insects travel more easily in autumn

C. insects never position themselves when flying low

D. insects rest a lot when the wind pushes them along

5. According to the passage, the findings can _____. [ ]

A. increase insects in number 

B. instruct farmers when to spray

C. prevent climate warming

D. help protect insects