问题 解答题

已知函数f(x)=ax+b,当x∈[a1,b1]时f(x)的值域为[a2,b2],当x∈[a2,b2]时f(x)的值域为[a3,b3],…依此类推,一般地,当x∈[an-1,bn-1]时f(x)的值域为[an,bn],其中a、b为常数且a1=0,b1=1

(1)若a=1,求数列{an},{bn}的通项公式.

(2)若a>0且a≠1,要使数列{bn}是公比不为1的等比数列,求b的值.

(3)若a<0,设数列{an},{bn}的前n项和分别为Sn,Tn,求(T1+T2+…+T2000)-(S1+S2+…+S2000)的值.

答案

(1)a=1时,f(x)=x+b在R上是增函数,

由已知,当n≥2时,x∈[an-1,bn-1],f(x)的值域是[an,bn],

∴an=f(an-1)=an-1+b,bn=f(bn-1)=bn-1+b,

∴{an}、{bn}都是公差为b的等差数列.

∵a1=0,b1=1,

∴an=(n-1)b,bn=(n-1)b+1;

(2)∵a>0,a≠1,

∴f(x)=ax+b在R上也是增函数,

由已知有bn=f(bn-1)=abn-1+b,即bn=abn-1+b(n≥2),

bn
bn-1
=a+
b
bn-1

若{bn}是公比不为1的等比数列,则

b
bn-1
是常数,所以b=0;

(3)∵a<0,∴f(x)=ax+b在R上是减函数,

由已知可得,bn=f(an-1)=a•an-1+b,an=f(bn-1)=a•bn-1+b,

∴bn-an=-a(bn-1-an-1)(n≥2),

∴{bn-an}是以1为首项,-a为公比的等比数列,

∴bn-an=(-a)n-1

∴Tn-Sn=(b1-a1)+(b2-a2)+…+(bn-an)=

n,a=-1
1-(-a)n
1+a
,a≠-1

于是,(T1+T2+…+T2000)-(S1+S2+…+S2000

=(T1-S1)+(T2-S2)+…+(T2000-S2000

=

2001000,a=-1
2000+2001a-a2001
(1+a)2
,a<0,a≠-1

阅读理解

阅读短文,回答问题。

         Jason Queally is one of the fastest men in the world on bicycle. But do you really call the thing in the

picture a bicycle? Well, yes, Jason's human-powered machine, With its two wheels, is, of course, a bicycle.

         Every year, a very important humanpowered bicycle is held in Nevada, USA.  The speed of a bike is

measured (测定) for only 200-metres, but players take more than a kilometre to get their bikes going fast. Jason

Queally's fastest speed for the 200metre race was 103.5 kilometres an hour.

         At this year's race, Jason failed to reach the finishing line. He was speeding along at about 70 kilometres

an hour when he began to lose control of his bike. When he tried to slow down, it began to smoke.Soon the

inside of his bike was filled with smoke.He couldn't see, and he couldn't breathe. At 70 kilometres an hour, a

crash could be very serious. Jason was frightened, but he managed to stop the bike safely. He would rapair his

bicycle and try again another time to be the world's fastest man on a bike. Better luck next time, Jason. 

         Maybe you're surprised that these bikes go so quickly, but is it useful? It could be. Cars are becoming

more and more popular, and they are very safe, comfortable and easy to drive, but they also pollute the air in

our cities. Scientists and engineers are learning from fast racing bikes how to make human-powered vehicles

that might be uesful for daily travel.

1. Is Jason's human-powered machine a bicycle?

                                                                                      

2. How often is the human-powered bicycle race held in Nevada?

   ________________________________________________________________

3. Did Jason stop his bike safely or was he badly hurt?

    ________________________________________________________________

4. Why did he lose this year's race? 

                                                            

5. What is the best vehicle for daily travel like according to the writer?

    ________________________________________________________________

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