解: (1)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823161300317843.gif)
--------1分
得
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823161300411516.gif)
-----------------3分
当
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823161300427395.gif)
时,
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823161300473253.gif)
不合题意舍去-----------------------------------4分
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823161300177234.gif)
时,带入
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823161300598486.gif)
可得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823161300614320.gif)
---------------------------5分
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823161300645381.gif)
构成以
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823161259584244.gif)
为首项,公差为 -1 的等差数列;
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823161300676422.gif)
--- -- ----6分
2)由
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823161300052338.gif)
可得,
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823161300723500.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823161300739472.gif)
,
就有
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823161300785720.gif)
,-------------------------------------------------.8分
即
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823161300801423.gif)
,
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823161300817414.gif)
,又
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823161300848528.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823161300863374.gif)
构成以
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823161300879294.gif)
为首项,公比为3 的等比数列;
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823161300910606.gif)
-------------------------------------------------------------------10分
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823161300941891.gif)
----------------------------------------------12分
(若由
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823161300177234.gif)
时,直接得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823161300973552.gif)
;即
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823161300988244.gif)
时,
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823161301019420.gif)
恒成立,
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823161300645381.gif)
构成以
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823161259584244.gif)
为首项,公差为1 的等差数列;
则
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823161301082669.gif)
. ………该解法不严谨本小题扣2分)