数列函数
来源:学生作业帮 编辑:作业帮 分类:数学作业 时间:2024/07/17 14:54:32
![](http://img.wesiedu.com/upload/6/01/6010c0948381b9d02d560c5e6d6c5d29.jpg)
解题思路: 主要考查你对 等比数列的定义及性质,二次函数的性质及应用,数列求和的其他方法(倒序相加,错位相减,裂项相加等) 等考点的理解
解题过程:
解:(1)由(an+1﹣an)g(an)+f(an)=0
得4(an+1﹣an)(an﹣1)+(an﹣1)2=0
化得:(an﹣1)(4an+1﹣4an+an﹣1)=0,?an﹣1=0或4an+1﹣4an+an﹣1=0,
由已知a1=2,∴an﹣1=0(舍去).
∴4an+1﹣4an+an﹣1=0得4an+1=3an+1
从而有:an+1﹣1=![](http://img.wesiedu.com/upload/6/5e/65ea68a8ccd92abb3f6140e68205b28a.png)
∴数列{an﹣1}是首项为a1﹣1=1,公比为
的等比数列
∴an﹣1=
,
∴数列{an}通项公式为an=
+1.
(2)由(1)知
=![](http://img.wesiedu.com/upload/6/32/6324d8584599d017e33a8f43ef1051c9.png)
+n=4[1﹣
]+n
∵对?n∈N*,有
,
∴![](http://img.wesiedu.com/upload/b/49/b49dafffdcc7fc8ef17eaf13ad31f82b.png)
,
∴
+n≥1+n,
即![](http://img.wesiedu.com/upload/4/69/4694eb50cd35c5b2140193cfac7693f1.png)
(3)由bn=3f(an)﹣g(an+1)得bn=3(an﹣1)2﹣4(an+1﹣1)
∴![](http://img.wesiedu.com/upload/d/60/d60821b60b8c91ad2fced5efd0d52aaa.png)
=![](http://img.wesiedu.com/upload/0/46/0463855ac9a96b4f7afdd468978045fa.png)
![](http://img.wesiedu.com/upload/c/64/c64b01608e16308d061c7f19654644f0.png)
令
,则0<u≤1,
bn=3(u2﹣u)=![](http://img.wesiedu.com/upload/3/07/30751ae42ea1be9322d7e96c9e347d07.png)
∵函数![](http://img.wesiedu.com/upload/f/35/f35255a263b28baa0ba7039f6ce52e1c.png)
在
上为增函数,在
上为减函数
当n=1时u=1,
当n=2时
,
当n=3时,
=
,
当n=4时
,
∵![](http://img.wesiedu.com/upload/7/6a/76ae1e59d7128a86a739e9f17f7e62f3.png)
,且![](http://img.wesiedu.com/upload/8/6c/86c6d3e71a19303fb2f0748bf0f8ad3a.png)
![](http://img.wesiedu.com/upload/5/84/58407e7368e74cc4587e3f214e48abaa.png)
∴当n=3时,bn有最小值,即数列{bn} 有最小项,最小项为![](http://img.wesiedu.com/upload/c/b6/cb67dc12a9adbeb5bfc1c0bab22cfe7c.png)
![](http://img.wesiedu.com/upload/a/4c/a4c15c52c962c507ce7010fdc5034a1c.png)
当n=1即u=1时,bn有最大值,即有最大项,最大项为b1=3(1﹣1)=0.
解题过程:
解:(1)由(an+1﹣an)g(an)+f(an)=0
得4(an+1﹣an)(an﹣1)+(an﹣1)2=0
化得:(an﹣1)(4an+1﹣4an+an﹣1)=0,?an﹣1=0或4an+1﹣4an+an﹣1=0,
由已知a1=2,∴an﹣1=0(舍去).
∴4an+1﹣4an+an﹣1=0得4an+1=3an+1
从而有:an+1﹣1=
![](http://img.wesiedu.com/upload/6/5e/65ea68a8ccd92abb3f6140e68205b28a.png)
∴数列{an﹣1}是首项为a1﹣1=1,公比为
![](http://img.wesiedu.com/upload/3/92/3926aa255ea99fdeb213cb8659b80ec6.png)
∴an﹣1=
![](http://img.wesiedu.com/upload/6/c8/6c8af94c2f7ebc3e498b9b90f7a53bb7.png)
∴数列{an}通项公式为an=
![](http://img.wesiedu.com/upload/8/02/80299e8b7a29f903b86af7cb70074248.png)
(2)由(1)知
![](http://img.wesiedu.com/upload/9/5c/95cdce1997e78105fbbb726b538b4a5e.png)
![](http://img.wesiedu.com/upload/6/32/6324d8584599d017e33a8f43ef1051c9.png)
![](http://img.wesiedu.com/upload/0/89/08967160ff8718d63d022f2a6e5d722c.png)
![](http://img.wesiedu.com/upload/8/26/826f5a4c0a29ae690389ffad88cc7190.png)
∵对?n∈N*,有
![](http://img.wesiedu.com/upload/9/22/922f74e1750053d5ece83e4d8cebf8fa.png)
∴
![](http://img.wesiedu.com/upload/b/49/b49dafffdcc7fc8ef17eaf13ad31f82b.png)
![](http://img.wesiedu.com/upload/7/90/7906c9e72c92c6789f6d8d20fa5b7ad1.png)
∴
![](http://img.wesiedu.com/upload/7/61/76151776117e70dd1bfc0bb3375a3437.png)
即
![](http://img.wesiedu.com/upload/4/69/4694eb50cd35c5b2140193cfac7693f1.png)
(3)由bn=3f(an)﹣g(an+1)得bn=3(an﹣1)2﹣4(an+1﹣1)
∴
![](http://img.wesiedu.com/upload/d/60/d60821b60b8c91ad2fced5efd0d52aaa.png)
![](http://img.wesiedu.com/upload/b/3d/b3d553d93ecebb74675ec150480be3ca.png)
![](http://img.wesiedu.com/upload/0/46/0463855ac9a96b4f7afdd468978045fa.png)
![](http://img.wesiedu.com/upload/c/64/c64b01608e16308d061c7f19654644f0.png)
令
![](http://img.wesiedu.com/upload/1/c9/1c9a3cdad89c44b9227a5bfe31022030.png)
bn=3(u2﹣u)=
![](http://img.wesiedu.com/upload/3/07/30751ae42ea1be9322d7e96c9e347d07.png)
∵函数
![](http://img.wesiedu.com/upload/f/35/f35255a263b28baa0ba7039f6ce52e1c.png)
![](http://img.wesiedu.com/upload/f/fe/ffec302e88fd2880c64fd41f76fe0d76.png)
![](http://img.wesiedu.com/upload/2/4f/24fc4ec392a81c0e8a27229b0dc32416.png)
![](http://img.wesiedu.com/upload/d/e8/de84d6e0a80317c1471cc20388468e55.png)
当n=1时u=1,
当n=2时
![](http://img.wesiedu.com/upload/9/ec/9ec7a14524d72d7e34930b9861c86aa0.png)
当n=3时,
![](http://img.wesiedu.com/upload/0/59/059040364356bab059eac2a7da93cf6e.png)
![](http://img.wesiedu.com/upload/7/62/762b7b4e0ad641f5753943c602f7762b.png)
当n=4时
![](http://img.wesiedu.com/upload/4/e3/4e3d4d89a81f3c56d6b7ad9b6c63bc65.png)
∵
![](http://img.wesiedu.com/upload/7/6a/76ae1e59d7128a86a739e9f17f7e62f3.png)
![](http://img.wesiedu.com/upload/f/eb/febaba41e450035f8c13a8b38d7ec81d.png)
![](http://img.wesiedu.com/upload/8/6c/86c6d3e71a19303fb2f0748bf0f8ad3a.png)
![](http://img.wesiedu.com/upload/5/84/58407e7368e74cc4587e3f214e48abaa.png)
∴当n=3时,bn有最小值,即数列{bn} 有最小项,最小项为
![](http://img.wesiedu.com/upload/c/b6/cb67dc12a9adbeb5bfc1c0bab22cfe7c.png)
![](http://img.wesiedu.com/upload/a/4c/a4c15c52c962c507ce7010fdc5034a1c.png)
当n=1即u=1时,bn有最大值,即有最大项,最大项为b1=3(1﹣1)=0.