若等比數(shù)列
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824035239788473.png)
的前
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824035239804277.png)
項和
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824035239820634.png)
則
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824035239835278.png)
等于( )
A.![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824035239866361.png) | B.![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824035239882339.png) | C.-1 | D.1 |
試題分析:由
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824035239820634.png)
得
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824035239913961.png)
,所以
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824035239929528.png)
,因此
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824035239944474.png)
;本題也可利用等比數(shù)列
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824035239788473.png)
的前
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824035239804277.png)
項和公式的特征:
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824035239991729.png)
,所以
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824035239944474.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824035240022388.png)
求
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824035240038348.png)
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