設
![](http://thumb.zyjl.cn/pic2/upload/papers/20140824/20140824012057750388.png)
是等差數列
![](http://thumb.zyjl.cn/pic2/upload/papers/20140824/20140824012057765480.png)
的前
![](http://thumb.zyjl.cn/pic2/upload/papers/20140824/20140824012057765297.png)
項和,若
![](http://thumb.zyjl.cn/pic2/upload/papers/20140824/20140824012057781509.png)
,則
![](http://thumb.zyjl.cn/pic2/upload/papers/20140824/20140824012057796355.png)
___________。
試題分析:由等差數列求和公式可知
![](http://thumb.zyjl.cn/pic2/upload/papers/20140824/201408240120578121201.png)
點評:等差數列求和公式
![](http://thumb.zyjl.cn/pic2/upload/papers/20140824/20140824012057828891.png)
,常用的重要性質:若
![](http://thumb.zyjl.cn/pic2/upload/papers/20140824/20140824012057843626.png)
則
![](http://thumb.zyjl.cn/pic2/upload/papers/20140824/20140824012057859672.png)
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