已知数列
的前n项和为
,
,
,则![](https://staticzujuan.xkw.com/quesimg/Upload/formula/c5a4adedeb2914234d502a896c098902.png)
_______ .
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/83cf38189d5cbf627d2b82ac0eb76006.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/08eb71ecf8d733b6932f4680874dbbf3.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/b065334d8f60c49f4bd3d9f1373fe4cd.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/d38228a6b9c040f171e6a497208e160f.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/c5a4adedeb2914234d502a896c098902.png)
更新时间:2021-05-13 23:29:22
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【知识点】 分组(并项)法求和
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【推荐1】已知数列{an}的通项公式为an=
·3n-1,n∈N*,[an]表示不超过an的最大整数(如[1.2]=1).记bn=[an],数列{bn}的前n项和为Tn,若不等式λan+1>Tn+5n-
对任意的n∈N*恒成立,则实数λ的取值范围是________ .
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/f89eef3148f2d4d09379767b4af69132.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/ce0709638fc38d6b880eac3c1c70c271.png)
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适中
(0.65)
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【推荐2】已知数列
满足
,则
的前20项和![](https://staticzujuan.xkw.com/quesimg/Upload/formula/3bc625e19e7ca2b9d097f67a3d472e47.png)
___________ .
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/63d471926f7b27322d90c82b9ce21d3d.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/37caa5ac410375207861e936b02262c4.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/63d471926f7b27322d90c82b9ce21d3d.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/3bc625e19e7ca2b9d097f67a3d472e47.png)
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名校
解题方法
【推荐3】为提升同学们的科创意识,学校成立社团专门研究密码问题,社团活动室用一把密码锁,密码一周一换,密码均为
的小数点后前6位数字,设定的规则为:
①周一至周日中最大的日期为x,如周一为3月28日,周日为4月3日,则取周四的3月31日的31作为x,即
;
②若x为偶数,则在正偶数数列中依次插入数值为
的项得到新数列
,即
,
,
,
,
,
,10,12,14,…;若x为奇数,则在正奇数数列中依次插入数值为
的项得到新数列
,即1,
,3,
,5,7,
,9,11,13,…;
③N为数列
的前x项和,如
,则9项分别为1,
,3,
,5,7,
,9,11,故
,因为
,所以密码为142857.
若周一为4月22日,则周一到周日的密码为____________ .
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/2fce9a31bd9c17cfbdb7633eb3f8fbb8.png)
①周一至周日中最大的日期为x,如周一为3月28日,周日为4月3日,则取周四的3月31日的31作为x,即
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/ebd5bb1faadcc09c1f47eb033c936422.png)
②若x为偶数,则在正偶数数列中依次插入数值为
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/7a332cbe8b05756beffe0c1e3a25de3a.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/83cf38189d5cbf627d2b82ac0eb76006.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/61128ab996360a038e6e64d82fcba004.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/3afa44962d486b748dc9bf8e9c85e397.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/b8860d9787671b53b1ab68b3d526f5ca.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/6f8c4c029e552954bd493b49aeab82d5.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/cd3304e23f3b0f9569c4140ca89b6498.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/bb3f00a02e931a265f86950296602f0f.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/f31971306914638e5ceb1bbe437535d3.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/83cf38189d5cbf627d2b82ac0eb76006.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/78697a69758b8d469a74236859514a72.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/b2ad4668cc927e277289b2af718f0d91.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/6648bc986a558fa32e752d28d3a68431.png)
③N为数列
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/83cf38189d5cbf627d2b82ac0eb76006.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/ea3680f7d0d96efc5b207c8e9552e21c.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/78697a69758b8d469a74236859514a72.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/b2ad4668cc927e277289b2af718f0d91.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/6648bc986a558fa32e752d28d3a68431.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/6809be2f0702a52f08c87fb3ab1559c2.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/864a98265cabc4fccc5a58cc5c52cc7a.png)
若周一为4月22日,则周一到周日的密码为
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