试题分析:(Ⅰ)当
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824024449148347.png)
时,求函数
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824024449164447.png)
的单调区间,首先确定定义域
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824024449351641.png)
,可通过单调性的定义,或求导确定单调区间,由于
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824024449133930.png)
,含有对数函数,可通过求导来确定单调区间,对函数
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824024449164447.png)
求导得
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824024449414695.png)
,由此令
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824024449445570.png)
,
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824024449460560.png)
,解出
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824024449476266.png)
就能求出函数
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824024449164447.png)
的单调区间;(Ⅱ)若
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824024449195705.png)
,对定义域内任意
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824024449476266.png)
,均有
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824024449211570.png)
恒成立,求实数
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824024449570283.png)
的取值范围,而
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408240244495851352.png)
,对定义域内任意
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824024449476266.png)
,均有
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824024449211570.png)
恒成立,属于恒成立问题,解这一类题,常常采用含有参数
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824024449570283.png)
的放到不等式的一边,不含参数
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824024449570283.png)
(即含
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824024449476266.png)
)的放到不等式的另一边,转化为函数的最值问题,但此题用此法比较麻烦,可考虑求其最小值,让最小值大于等于零即可,因此对函数
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824024449694484.png)
求导,利用导数确定最小值,从而求出
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824024449570283.png)
的取值范围;(Ⅲ)由(Ⅱ)知,当
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824024449726472.png)
时,
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408240244497571057.png)
,当且仅当
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824024449772323.png)
时,等号成立,这个不等式等价于
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824024449788518.png)
,即
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824024449804842.png)
,由此对任意的正整数
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824024449226435.png)
,不等式
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408240244498502051.png)
恒成立.
试题解析:(Ⅰ)定义域为(0,+∞),
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824024449866994.png)
,
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408240244498821908.png)
,所以
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824024449164447.png)
在
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408240244492731104.png)
(4分)
(Ⅱ)
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408240244499442218.png)
,当
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824024449960387.png)
时,
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824024449694484.png)
在
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824024449991348.png)
上递减,在
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824024450006431.png)
上递增,
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824024450022974.png)
,当
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824024450053402.png)
时,
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824024450053788.png)
不可能成立,综上
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/20140824024449289507.png)
;(9分)
(Ⅲ)令
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408240244500841818.png)
,
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408240244501002572.png)
相加得到
![](http://thumb.1010pic.com/pic2/upload/papers/20140824/201408240244501162079.png)
得证。(14分)