题目内容
已知函数
(
是不为零的实数,
为自然对数的底数).
(1)若曲线
与
有公共点,且在它们的某一公共点处有共同的切线,求k的值;
(2)若函数
在区间
内单调递减,求此时k的取值范围.
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020236359616.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020236375312.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020236390264.png)
(1)若曲线
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020236406562.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020236422467.png)
(2)若函数
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020236437945.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020236453530.png)
(1)
.
(2)当
时,函数
在区间
内单调递减.
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020236468529.png)
(2)当
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020236484645.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020236500937.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020236453530.png)
试题分析:(1)设曲线
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020236406562.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020236546466.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020236562642.png)
则
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020236609588.png)
又曲线
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020236406562.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020236546466.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020236562642.png)
且
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020236671694.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020236687568.png)
∴
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020236702676.png)
解得
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020236468529.png)
(2)由
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020236359616.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020236749943.png)
所以
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408240202367801265.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408240202367961120.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020236812993.png)
又由区间
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020236453530.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020236843469.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020236858455.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020236874390.png)
①当
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020236858455.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020236968581.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408240202369831050.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020236999661.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020237014484.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020237077628.png)
要使得函数
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020236500937.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020236453530.png)
则有
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408240202371391238.png)
解得
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020236484645.png)
②当
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020236874390.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020236968581.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408240202369831050.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020237233481.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020237248561.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020237014484.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020237280604.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020237295676.png)
要使得函数
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020236500937.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020236453530.png)
则有
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020237358806.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020237498784.png)
这两个不等式组均无解. 13分
综上,当
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020236484645.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020236500937.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824020236453530.png)
点评:难题,本题属于导数内容中的基本问题,(1)运用“函数在某点的切线斜率,就是该点的导数值”,确定直线的斜率。通过研究导数值的正负情况,明确函数的单调区间。确定函数的最值,往往遵循“求导数,求驻点,计算极值、端点函数值,比较大小确定最值”。本题较难,主要是涉及参数K的分类讨论,不易把握。
![](http://thumb2018.1010pic.com/images/loading.gif)
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