题目内容
(12分)设集合W是满足下列两个条件的无穷数列{an}的集合:
①
②
,其中n∈N*,M是与n无关的常数
(1)若{an}是等差数列,Sn是其前n项的和,a3=4,S3=18,试探究{Sn}与集合W之间的关系;
(2)设数列{bn}的通项为bn=5n-2n,且{bn}∈W,M的最小值为m,求m的值;
(3)在(2)的条件下,设
,求证:数列{Cn}中任意不同的三项都不能成为等比数列.
①
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823220028210742.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823220028225579.png)
(1)若{an}是等差数列,Sn是其前n项的和,a3=4,S3=18,试探究{Sn}与集合W之间的关系;
(2)设数列{bn}的通项为bn=5n-2n,且{bn}∈W,M的最小值为m,求m的值;
(3)在(2)的条件下,设
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408232200282411128.png)
(1) {Sn}
W ; (2) M的最小值为7; (3) 见解析.
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823220028256228.png)
第一问利用Sn=-n2+9n
满足①
当n=4或5时,Sn取最大值20
第二问中bn+1-bn=5-2n可知{bn}中最大项是b3=7
∴ M≥7 M的最小值为7 …………8分
第三问中
,假设{Cn}中存在三项bp、bq、br(p、q、r互不相等)
成等比数列,则bq2=bp·br
∴![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823220028412972.png)
∴![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823220028444999.png)
∵ p、q、r∈N*
∴ p=r与p≠r矛盾
解:(1) Sn=-n2+9n
满足①
当n=4或5时,Sn取最大值20
∴Sn≤20满足② ∴{Sn}∈W …………4分
(2) bn+1-bn=5-2n可知{bn}中最大项是b3=7
∴ M≥7 M的最小值为7 …………8分
(3)
,假设{Cn}中存在三项bp、bq、br(p、q、r互不相等)
成等比数列,则bq2=bp·br
∴![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823220028412972.png)
∴![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823220028444999.png)
∵ p、q、r∈N*
∴ p=r与p≠r矛盾
∴ {Cn}中任意不同的三项都不能成为等比数列 …………12分
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823220028350785.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408232200283661023.png)
第二问中bn+1-bn=5-2n可知{bn}中最大项是b3=7
∴ M≥7 M的最小值为7 …………8分
第三问中
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823220028381629.png)
成等比数列,则bq2=bp·br
∴
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823220028412972.png)
∴
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823220028444999.png)
∵ p、q、r∈N*
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823220028475957.png)
∴ p=r与p≠r矛盾
解:(1) Sn=-n2+9n
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823220028350785.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408232200283661023.png)
∴Sn≤20满足② ∴{Sn}∈W …………4分
(2) bn+1-bn=5-2n可知{bn}中最大项是b3=7
∴ M≥7 M的最小值为7 …………8分
(3)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823220028381629.png)
成等比数列,则bq2=bp·br
∴
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823220028412972.png)
∴
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823220028444999.png)
∵ p、q、r∈N*
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823220028475957.png)
∴ p=r与p≠r矛盾
∴ {Cn}中任意不同的三项都不能成为等比数列 …………12分
![](http://thumb2018.1010pic.com/images/loading.gif)
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