问题 解答题

已知函数f(x)=ex•g(x),其中g(x)=ax2-2x-2.

(1)若存在x∈R,使得g(x)>0成立,求实数a的取值范围;

(2)求函数y=f(|sinx|)的值域.

答案

(1)存在x∈R,使得g(x)>0,

即存在x∈R,使得ax2-2x-2>0,

当a>0时,满足要求;当a=0时,满足要求;

当a<0时,△>0,解得-

1
2
<a<0

综上得,a>-

1
2
(4分)

(2)f(x)=ex•g(x)=ex•(ax2-2x-2)

∴f′(x)=(ex)′•(ax2-2x-2)+ex•(ax2-2x-2)′

=ex•(ax2-2x-2)+ex•(2ax-2)

=ex•[ax2+(2a-2)x-4]

设|sinx|=t,(0≤t≤1),则转化为求函数y=f(t),(0≤t≤1)的值域.

当a=0时,f′(x)=-2ex•(x+2)<0,此时函数f(t)在[0,1]上为减函数,

∴函数f(t)的值域为[f(1),f(0)],即[(a-4)e,-2]

当a<0时,f(x)=ex•[ax2+(2a-2)x-4]=a•ex•(x-

2
a
)(x+2)<0

此时函数f(t)在[0,1]上为减函数,

∴函数f(t)的值域为[f(1),f(0)],即[(a-4)e,-2](6分)

当a>0时,f(x)=ex•[ax2+(2a-2)x-4]=a•ex•(x-

2
a
)(x+2)

令f′(x)=0,解得x=

2
a
或x=-2(舍).

当x变化时,f(x)与f′(x)的变化情况如下表:

2
a
≥1,即0<a≤2时,函数f(t)在[0,1]上为减函数.

∴函数f(t)的值域为[f(1),f(0)],即[(a-4)e,-2]

0<

2
a
<1,即a>2时,函数f(t)在(0,
2
a
)
上递减,在(
2
a
,1)
上递增

ymin=f(

2
a
)=-2e
2
a
函数f(t)在[0,1]上的最大值为f(0)与f(1)中的较大者

∵f(0)=-2,f(1)=(a-4)e,∴f(1)-f(0)=(a-4)e+2

∴当a>4-

2
e
时,f(1)>f(0),此时ymax=f(1)=(a-4)e;

a=4-

2
e
时,f(1)=f(0),此时ymax=f(0)=f(1)=-2;

2<a<4-

2
e
时,f(1)<f(0),此时ymax=f(0)=-2(13分)

综上,当a≤2时,函数f(|sinx|)的值域为[(a-4)e,-2];

2<a≤4-

2
e
时,函数f(|sinx|)的值域为[-2e
2
a
,-2]

a>4-

2
e
时,函数f(|sinx|)的值域为[-2e
2
a
,(a-4)e]
.(14分)

阅读理解

阅读理解。

    The American expression "shoo-in" means someone or something that seems sure to win a race or

competition.

    Before a big football game, supporters of each are certain to argue (争论) about which side will win.

    Sometimes, however, one team appears so much stronger than the others that everyone agrees it will win.

The stronger team is a shoo-in. there is no way that it can lose.

    The expression "shoo-in" comes from horse racing. It comes from the word "shoo". That is not the shoe

we wear on our feet. It's another word, S-H-O-O. To shoo is an expression hundreds of years old that means

to force an animal to move in a desired direction.

    Many years ago dishonest (不诚实) riders sometimes agreed secretly to control their horses so that one

chosen horse would win the race. All but one of the riders would hold back their horses. The chosen rider

would shoo his horse ahead of the others, and win the race.

     The other riders would secretly bet (打赌) large amounts of money that the chosen horse would win. The

public (公众) soon learned about such races. They began to call the winner of such a race a "shoo-in".

     These days, people use shoo-into describe (描述) any athlete (运动员) or competitor that seems certain to

win, even without cheating.

      American runner Edewin Moses won more than hundred races before he ran the hurdle race (跳拦赛) in

the 1984 Olympic Games. Everyone said Moses was a shoo-in, and that he could not lose. And they were

right. He won the gold medal.

1. One of the dishonest riders was sure to win the race because ________.

A. be used to be the best in riding

B. the public had belief in him

C. he could not control his horse easily

D. the other riders had agreed to fail in the race

2. The reason why American runner Edewin Moses was regarded as a "shoo-in" is ________.

A. due to his honest in the races he ran in

B. due to his great effort (努力) shown in more than one hundred races

C. that he won over 100 competitions before the 1984 Olympic Games

D. that the old meaning of "shoo-in" had changed

3. Which of the following statements is not true?

A. "shoo-in" came from the word "shoo".

B. The US runner Moses was worthy of (配的上) a shoo-in.

C. The public did not know they were being cheated.

D. Some runners bet a lot of money on the chosen horse.

4. The main idea of the passage is that "shoo-in" is ________.

A. used for anyone or anything that is certain to win a game

B. used to admire (钦佩) a person who seems to be sure to win a race

C. a word used to describe a dishonest person

D. used for any athlete or competitor that seems too clever to be cheated

5. In the last paragraph the author means to tell us ________.

A. who Edewin Moses was

B. Moses won a lot of races

C. Moses was the best runner in America

D. We can use "shoo-in" to describe an excellent runner

单项选择题