题目内容
(本题满分10分)已知向量
,求
(Ⅰ)
;
(Ⅱ)若
的最小值是
,求实数
的值.
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408231957058091881.png)
(Ⅰ)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823195705825637.png)
(Ⅱ)若
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823195705841932.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823195705887417.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823195705903325.png)
解:(Ⅰ)(5分) a·b=
------------------2分
| a+b|=
-----2分
∵
, ∴
∴| a+b|=2cosx.-----------------------------------------------------------------------1分
(Ⅱ)(5分)
即
------------------------------------------------2分
∵
, ∴![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823195706402582.png)
时,当且仅当
取得最小值-1,这与已知矛盾.
时,当且仅当
取最小值![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823195706589508.png)
由已知得
,解得![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823195706636515.png)
时,当且仅当
取得最小值![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823195706699469.png)
由已知得
,解得
,这与
相矛盾.
综上所述,
为所求.----------------3分
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408231957059191151.png)
| a+b|=
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408231957059502247.png)
∵
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823195705965684.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823195706184543.png)
∴| a+b|=2cosx.-----------------------------------------------------------------------1分
(Ⅱ)(5分)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823195706324899.png)
即
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408231957063401023.png)
∵
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823195705965684.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823195706402582.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823195706496553.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823195706527744.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823195706543648.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823195706558781.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823195706589508.png)
由已知得
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823195706605695.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823195706636515.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823195706652556.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823195706667717.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823195706699469.png)
由已知得
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823195706714659.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823195706730558.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823195706761411.png)
综上所述,
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823195706777499.png)
略
![](http://thumb2018.1010pic.com/images/loading.gif)
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