题目内容
(本小题满分13分)在△ABC中,满足
的夹角为
,M是AB的中点
(1)若
,求向量
的夹角的余弦值
(2)若
,在AC上确定一点D的位置,使得
达到最小,并求出最小值。
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823192510500363.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823192510516254.gif)
(1)若
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823192510547442.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823192510563585.gif)
(2)若
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823192510578685.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823192510594477.gif)
(1)设![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408231925107192224.gif)
(2)因为
,由余弦定理可得:![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823192510937323.gif)
M是AB的中点,所以AM=1,因为D是AC上一点,设
,所以![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408231925109841398.gif)
=![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408231925110001179.gif)
所以当
时,即D距A点
处
取到最小,最小值为![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823192511249280.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408231925107192224.gif)
(2)因为
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408231925109221016.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823192510937323.gif)
M是AB的中点,所以AM=1,因为D是AC上一点,设
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823192510968600.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408231925109841398.gif)
=
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408231925110001179.gif)
所以当
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823192511015549.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823192511031223.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823192510594477.gif)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823192511249280.gif)
略
![](http://thumb2018.1010pic.com/images/loading.gif)
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