问题 解答题
设函数f(x)=(x-a)2,g(x)=x,x∈R,a为实常数.
(1)若a>0,设F(x)=
f(x)
g(x)
,x≠0,用函数单调性的定义证明:函数F(x)在区间[a,+∞)上是增函数;
(2)设关于x的方程f(x)=|g(x)|在R上恰好有三个不相等的实数解,求a的值所组成的集合.
答案

(1)F(x)=

x2-2ax+a2
x
=x+
a2
x
-2a,任取x1,x2∈[a,+∞),且x1<x2

F(x2)-F(x1)=x2-x1+a2(

1
x2
-
1
x1
)=(x2-x1)•
x1x2-a2
x1x2
,…(3分)

因为 a>0,x1≥a,x2≥a且x1<x2,∴x2-x1>0,x1x2>a2,…(4分)

所以F(x2)-F(x1)>0,所以函数F(x)在区间[a,+∞)上是增函数.…(6分)

(2)原方程为(x-a)2=|x|,

①当a=0时,原方程变为x2=|x|,有-1,0,1三个解;…(8分)

②当a<0时,函数y=(x-a)2与y=|x|的图象在x<0时有两个交点,所以原方程在x<0时有两个不相等的实数解,要使原方程在x>0时恰有一个解,当且仅当函数y=(x-a)2与y=|x|的图象在x>0时有且仅有一个公共点,即方程(x-a)2=x的判别式等于0,即(2a+1)2-4a2=0,解得a=-

1
4
;…(10分)

③同理,当a>0时,原方程在x>0时有两个不相等的实数解,要原方程在x<0时恰有一个解,当且仅当方程(x-a)2=-x的判别式等于0,即(2a-1)2-4a2=0,

解得a=

1
4
.…(12分)

综上,a的值所组成的集合为{-

1
4
,0,
1
4
}.…(14分)

阅读理解

阅读理解

       Many people like animals and raise dogs, cats or some kinds of birds. I love dogs most. My aunt

gave me a dog on my birthday. I called it Wang Wang. It is a little black dog. It is friendly and helpful.

It likes to run and play with me. Sometimes it follows my father around in the field.

      One day, my father took off his grey coat and put it on the ground under a big tree. Wang Wang

stood watching him. My father said, " Watch over my coat, Wang Wang. " Wang Wang sat down beside

the coat. My father went to work. After he finished his work, he forgot all about his coat and went home. Later in the evening I can't find my dog. I looked for it everywhere calling, " Wang Wang!Wang Wang!"

But Wang Wang didn't come back. Soon my father wanted something that was in that coat pocket. Then

he remembered Wang Wang. He went back to the big tree. He was surprised but happy. Wang Wang

was sitting on the coat so that nobody could take it away .

1. The writer's favourite animal is_________.

A. cat    

B. dog    

C. bird    

D. pet

2. Wang Wang is            ____.  

A. helpful and friendly

B. cute and naughty

C. naughty and friendly

D. lovely but unfriendly

3. The dog is            ____.

A. grey    

B. black and white

C. white   

D. black

4. What did the writer's father ask Wang Wang to do under the tree?

A. To watch over the coat.

B. To walk around.

C. To follow him.

D. To play with him.

5. When the writer's father found Wang Wang, Wang Wang was sitting           ____.

A. under the small tree

B. on the coat

C. in the field

D. beside the coat

单项选择题