题目内容
在数列{an}中,a1=
,an=1-
(n≥2,n∈N*),数列{an}的前n项和为Sn.
(1)求证:an+3=an;(2)求a2 008.
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823130023599206.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823130023614252.gif)
(1)求证:an+3=an;(2)求a2 008.
(1)证明见解析(2)a2 008=![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823130023599206.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823130023599206.gif)
(1)证明 an+3=1-
=1-![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823130023677424.gif)
=1-
=![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823130023708400.gif)
=1-
=1-![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823130023739383.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/2014082313002375573.gif)
=1-
=1-(1-an)=an.
∴an+3=an.
(2)解 由(1)知数列{an}的周期T=3,
a1=
,a2=-1,a3=2.
又∵a2 008=a3×669+1=a1=
.∴a2 008=
.
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823130023661365.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823130023677424.gif)
=1-
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823130023692347.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823130023708400.gif)
=1-
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823130023723342.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823130023739383.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/2014082313002375573.gif)
=1-
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823130023770298.gif)
∴an+3=an.
(2)解 由(1)知数列{an}的周期T=3,
a1=
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823130023599206.gif)
又∵a2 008=a3×669+1=a1=
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823130023599206.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823130023599206.gif)
![](http://thumb2018.1010pic.com/images/loading.gif)
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