等差数列
中,已知
,那么
的值是_________ .
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/83cf38189d5cbf627d2b82ac0eb76006.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/e9bc693d75f7918a49cae6d075da2aca.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/5f3cf6f2bbe20a404fea41a4d2b1c4c7.png)
更新时间:2016-12-04 02:48:29
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【知识点】 等差数列
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【推荐1】已知等差数列
的前
项和为
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,
,则
的值为____ .
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/63d471926f7b27322d90c82b9ce21d3d.png)
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【推荐2】变径圆弧螺旋线是以不同半径的圆弧连接而成的螺旋线,这种螺旋线极具美感.图1是鹦鹉螺的截面,其轮廓是等比变径螺旋线(半径构成等比数列),图2是一段等差变径圆弧螺旋线(半径构成等差数列),其中ABCDEF是边长为1的正六边形,弧
是以A为圆心,AF为半径的圆弧,弧
是以B为圆心,
为半径的圆弧,弧
是以C为圆心,
为半径的圆弧,依次类推,已知各圆弧的圆心角均等于正六边形的外角,则弧
的长为_________ .
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/3ed31a2625decb543f193a8653a96669.png)
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