题目
题型:不详难度:来源:
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005540-30985.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005540-30841.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005541-80864.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005541-47506.png)
(Ⅰ)求
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005541-57483.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005541-80856.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005541-56281.png)
(Ⅱ)记数列{an}前2n项和为
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005542-70325.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005542-70325.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005542-10732.png)
答案
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005542-93936.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005542-40047.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005542-70447.png)
解析
试题分析:本题考查等差数列的通项公式和前
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005542-10732.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005542-10732.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005542-10732.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005542-10732.png)
试题解析:(I)∵
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005543-98724.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005541-80864.png)
∴
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005542-93936.png)
由题意可得数列
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005543-72511.png)
当
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005542-10732.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005543-75837.png)
当
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005542-10732.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005559-90818.png)
∴
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005542-40047.png)
(II)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005600-61794.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005600-32951.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005600-94772.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005600-11127.png)
结合二次函数的性质可知,当
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005542-70447.png)
核心考点
举一反三
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005531-26050.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005532-76659.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005532-27611.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005532-86176.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005532-69789.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005532-59313.png)
A.![]() | B.![]() | C.![]() | D.![]() |
(Ⅰ)求a3,a4,并求数列{an}通项公式;
(Ⅱ)记数列{an}前2n项和为S2n,当S2n取最大值时,求n的值.
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005448-89885.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005448-10937.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005448-45833.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005449-73637.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005449-45012.png)
A.![]() | B.![]() | C.![]() | D.![]() |
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005442-47887.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005443-14572.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005442-47887.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005443-47499.png)
A.63 | B.109 | C.117 | D.210 |
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005428-17248.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005429-41341.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005429-53633.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005429-81716.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191012/20191012005429-45103.png)
A.![]() | B.![]() | C.![]() | D.![]() |
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