问题 选择题
下列4个命题:
(1)若a<b,则am2<bm2
(2)“a≤2”是“对任意的实数x,|x+1|+|x-1|≥a成立”的充要条件;
(3)命题“∃x∈R,x2-x>0”的否定是:“∀x∈R,x2-x<0”;
(4)函数f(x)=
2x-1
2x+1
的值域为[-1,1].
其中正确的命题个数是(  )
A.1B.2C.3D.0
答案

由于当m=0时,由a<b不能推出am2<bm2,可得①不正确

对于②,当a≤2时,不等式|x+1|+|x-1|≥|(x+1)-(x-1)|=2≥a恒成立.

当不等式|x+1|+|x-1|≥a恒成成立时,也可得到a≤2.

因此“a≤2”是“对任意的实数x,|x+1|+|x-1|≥a成立”的充要条件,故②正确;

对于③,命题“∃x∈R,x2-x>0”的否定是:“∀x∈R,x2-x≤0”,故③不正确;

对于④,令y=f(x)=

2x-1
2x+1
,可得2x=
1-y
1+y

由2x=

1-y
1+y
>0,解得y∈(-1,1],因此函数的值域为(-1,1],故④不正确

综上所述,只有②一个命题正确

故选:A

阅读理解

阅读理解。

     What will the weather be like? Some people say they can know what the weather will be like from 

birds. When they see birds fly high (高的) out to the sea. they know it is a nice day, because birds don't 

like to fly out when it will rain. When the weather will be bad, the birds come back to the beach. A big 

wind may come with a rain. Birds do not like to fly in a big wind. They may sit in a tree when rain is 

coming. Other animals can also tell the weather. It's a cloudy day. The rain is coming. Chickens aren't 

quiet, and they are running here and there. Frogs (青蛙) are making big noises in the pool. If it is a nice 

day, the frogs will come out of the water.

1. What's the best title (题目) for the passage (短文) about?

A. Weather and the animals.

B. Birds.

C. Sea.

D. Rain.

2. How many kinds of animals can we find in the passage?

A. One.        

B. Two.   

C. Three.   

D. Four.

3. We can play beach soccer when birds ____.

A. fly in the sky      

B. come back to the beach

C. play on the beach  

D. sit in a tree

4. When there is rain, frogs are not ____.

A. in the pool      

B. out of the water

C. making big noises  

D. swimming in the water

5. What does the underlined (画线的) word "tell" mean in the article (文章)?

A. 告诉        

B. 判断      

C. 说            

D. 谈话

单项选择题