试题分析:(1)利用
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824021519013564.png)
求出
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824021519029348.png)
与
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824021519044392.png)
的关系,判断数列是等差数列,从而写出等差数列的通项公式;(2)因为
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824021519060641.png)
,所以可以证明
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824021519076635.png)
是首项为
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824021519154338.png)
,公差为1的等差数列,先求出
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824021519169414.png)
的通项公式,再求
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824021519216365.png)
;(3)把第(2)问的
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824021519216365.png)
代入,利用错位相减法求
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824021518966373.png)
.
试题解析:(1)证明:当
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824021519247357.png)
时,
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824021519263768.png)
,解得
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824021519278371.png)
. 1分
当
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824021519419435.png)
时,
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824021519434789.png)
.即
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824021519450736.png)
. 2分
又
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824021518873337.png)
为常数,且
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824021519497431.png)
,∴
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408240215195121007.png)
.
∴数列
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824021519528456.png)
是首项为1,公比为
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824021519544456.png)
的等比数列. 3分
(2)解:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824021519559561.png)
. 4分
∵
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824021519060641.png)
,∴
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824021519606168.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824021519622625.png)
,即
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824021519637924.png)
. 5分
∴
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824021519076635.png)
是首项为
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824021519154338.png)
,公差为1的等差数列. 6分
∴
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824021519809991.png)
,即
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408240215189821013.png)
. 7分
(3)解:由(2)知
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824021519856678.png)
,则
所以
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408240215199021728.png)
8分
当
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824021518826297.png)
为偶数时,
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408240215199342369.png)
令
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408240215199491441.png)
①
则
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408240215199651892.png)
②
①-②得
=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408240215199961438.png)
=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408240215200121168.png)
=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824021520027998.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408240215200431130.png)
10分
令
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408240215200581159.png)
③
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408240215200741794.png)
④
③-④得
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408240215200901622.png)
=
=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408240215201211209.png)
=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408240215201361042.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408240215201521199.png)
11分
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408240215201682980.png)
12分
当
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824021518826297.png)
为奇数时,
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824021520199353.png)
为偶数,
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408240215202142347.png)
=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408240215202302500.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408240215202462635.png)
14分
法二:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408240215202771729.png)
①
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408240215202922366.png)
②
9分
①-②得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408240215203082507.png)
10分
=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408240215203241516.png)
12分
=
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408240215203391449.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408240215203552008.png)
13分
∴
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408240215203701138.png)
14分