题目内容
已知函数
在
有最大值5,
求实数
的值.
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408232139383041372.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823213938335669.png)
求实数
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823213938382337.png)
1.
解本题的关键是确定![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823213938398447.png)
之后
再令
,
则
,从而
,可以
令
进行二次换元降低思维难度,转化为二次函数基本题型来做即可.
解:![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408232139385231927.png)
令
,
则
,从而![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408232139384761307.png)
令
,由题意知
在
有最大值5.
当
时,
在
时有最大值5,故
符合条件;
当
时,
,矛盾!
当
时,
,矛盾!
综上所述,所求的实数
.
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823213938398447.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408232139384291121.png)
再令
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408232139384451402.png)
则
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823213938460763.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408232139384761307.png)
令
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823213938491656.png)
解:
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408232139385231927.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408232139385381147.png)
令
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408232139384451402.png)
则
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823213938460763.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408232139384761307.png)
令
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823213938491656.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823213938632918.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823213938647529.png)
当
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823213938679464.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823213938694622.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823213938725412.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823213938741386.png)
当
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823213938757503.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408232139387881055.png)
当
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823213938803487.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823213938835722.png)
综上所述,所求的实数
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823213938741386.png)
![](http://thumb2018.1010pic.com/images/loading.gif)
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