已知sinα+cosα=
![manfen5.com 满分网](http://img.manfen5.com/res/GZSX/web/STSource/20131103102747346714736/SYS201311031027473467147018_ST/0.png)
,
![manfen5.com 满分网](http://img.manfen5.com/res/GZSX/web/STSource/20131103102747346714736/SYS201311031027473467147018_ST/1.png)
,求cos(α+2β)的值.
考点分析:
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已知数列a
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(1)求证:数列a
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已知α为第二象限角,
![manfen5.com 满分网](http://img.manfen5.com/res/GZSX/web/STSource/20131103102747346714736/SYS201311031027473467147013_ST/0.png)
,则cos2α=
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