题目
题型:不详难度:来源:
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232943-44438.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232943-55124.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232943-27448.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232944-24533.gif)
(1)求通项公式an
(2)若
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232944-59526.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232944-19445.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232944-70328.gif)
答案
(1)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232944-95794.gif)
(2)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232945-49160.gif)
解析
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232944-24533.gif)
∴
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232945-35296.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232945-24532.gif)
又数列{an}是等差数列,
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232943-55124.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232946-27483.gif)
∴(
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232946-89711.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232946-16191.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232946-70634.gif)
解之得:
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232946-83340.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232947-31167.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232947-75989.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232947-22176.gif)
当
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232947-79017.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232947-29537.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232948-41954.gif)
∴
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232944-95794.gif)
⑵由⑴知
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232948-29429.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232948-81953.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232949-15397.gif)
∴
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232949-84737.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232949-43612.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232949-95511.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232945-49160.gif)
核心考点
举一反三
在数列
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232925-88879.gif)
(I)若
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232925-47411.gif)
(II)若
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232925-35751.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232925-66392.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232914-71177.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232914-50918.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232914-93341.png)
A.![]() | B.![]() | C.![]() | D.![]() |
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232902-49658.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232902-46374.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232902-23654.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232902-67480.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232903-35146.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232902-23654.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232903-72141.gif)
(1)取
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232903-13404.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232903-30739.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232904-81183.gif)
(2)当
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232904-41021.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232904-70877.gif)
(3)当
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232904-12898.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232902-23654.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232905-41221.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232905-64174.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232823-44894.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232824-54853.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232824-25019.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232824-67812.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232824-32528.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232825-89566.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232825-25611.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232825-71904.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232825-71904.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232825-16437.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232825-95040.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232826-40242.gif)
(1)求数列
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232824-32528.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232825-89566.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232826-66805.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232826-78620.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232825-95040.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232826-53874.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232825-25611.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232827-28711.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232816-92875.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232816-53530.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232817-16676.gif)
(Ⅰ)数列
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232817-48214.gif)
(Ⅱ)数列
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232817-16676.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232817-90633.gif)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012232817-71087.gif)
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