设
是公差不为0的等差数列
的前n项和,若
,则k =__________ .
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/70cb7b6d14630288595af4d9ad841312.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/cf3783e69ef5a6a0af566ff4e21ccf03.png)
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/5b497ada2a9115c51e2cb06e52942a33.png)
更新时间:2024-05-31 21:57:46
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解题方法
【推荐1】设等差数列
的前
项和为
,若
,则
=_______ .
![](https://img.xkw.com/dksih/QBM/2016/6/21/1572754529492992/1572754535735296/STEM/2b8447320ea742c09235024baa548059.png?resizew=31)
![](https://img.xkw.com/dksih/QBM/2016/6/21/1572754529492992/1572754535735296/STEM/1d6aba7c30ca4303a7145b9082c165b6.png?resizew=13)
![](https://img.xkw.com/dksih/QBM/2016/6/21/1572754529492992/1572754535735296/STEM/1b5b70d9fbb24381b6649581cd10aad7.png?resizew=20)
![](https://img.xkw.com/dksih/QBM/2016/6/21/1572754529492992/1572754535735296/STEM/b5499b1621c848b0a801417eced52da1.png?resizew=156)
![](https://img.xkw.com/dksih/QBM/2016/6/21/1572754529492992/1572754535735296/STEM/c44e16cf44bf4f6db841c85b205d97fb.png?resizew=23)
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【推荐2】已知各项不为0的等差数列{an}满足2a2-
+2a12=0,数列{bn}是等比数列,且b7=a7,则b3b11等于____________ .
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/57a52616446c7d86d7d216a522d7ac61.png)
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【推荐1】一般把数字出现的规律满足如图的模型称为蛇形模型:数字1出现在第1行;数字2,3出现在第2行;数字6,5,4(从左至右)出现在第3行;数字7,8,9,10出现在第4行,依此类推,则第21行从左至右的第4个数字应是____________ .
![](https://img.xkw.com/dksih/QBM/editorImg/2022/12/13/c30e49b2-faae-4137-9b61-d899ac879c88.png?resizew=225)
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【推荐2】已知等差数列
的公差为
,
,前
项和为
,则
的数值是 .
![](https://staticzujuan.xkw.com/quesimg/Upload/formula/abde9618774c7e9ba7cacb092cbb5b33.png)
![](https://img.xkw.com/dksih/QBM/2014/4/25/1571675504001024/1571675509432320/STEM/57b5ae82be1147e7aff21361c561753d.png?resizew=12)
![](https://img.xkw.com/dksih/QBM/2014/4/25/1571675504001024/1571675509432320/STEM/46878db729e14becb840f57204686879.png?resizew=49)
![](https://img.xkw.com/dksih/QBM/2014/4/25/1571675504001024/1571675509432320/STEM/24445061a44747728a4fd8b81dfaecf7.png?resizew=13)
![](https://img.xkw.com/dksih/QBM/2014/4/25/1571675504001024/1571675509432320/STEM/3d67d7745e9646dfa02a011a847c774a.png?resizew=19)
![](https://img.xkw.com/dksih/QBM/2014/4/25/1571675504001024/1571675509432320/STEM/611e36a34a5e49ff83825325e4909690.png?resizew=55)
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