题目内容
设平面内的向量
,
,
,点
是直线
上的一个动点,且
,求
的坐标及
的余弦值.
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823231128078623.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823231128093616.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823231128125720.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823231128140289.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823231128171467.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823231128203627.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823231128218381.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823231128234498.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823231128249652.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823231128374981.png)
本题考查了向量共线的条件,向量的坐标运算,数量积的坐标表示,向量的模的求法及利用数量积计算夹角的余弦,本题综合性强,运算量大,谨慎计算是正确解题的关键
(1)设
.
∵点
在直线
上,
∴
与
共线,而![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823231128702483.png)
,
∴
,即
,有![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823231128795702.png)
∴
又
,那么得到坐标,进而求解夹角的余弦值。
解:设
.
∵点
在直线
上,
∴
与
共线,而![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823231128702483.png)
,
∴
,即
,有
. ……………… 4分
∵
,
,
∴
,
即
. 又
, ∴
,
所以
,
,此时
. ……………………8分
.
于是
.
∴
………………12分
(1)设
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823231128624661.png)
∵点
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823231128140289.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823231128171467.png)
∴
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823231128218381.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823231128702483.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823231128702483.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823231128733478.png)
∴
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823231128749539.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823231128780484.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823231128795702.png)
∴
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408232311288111234.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823231128203627.png)
解:设
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823231128624661.png)
∵点
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823231128140289.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823231128171467.png)
∴
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823231128218381.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823231128702483.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823231128702483.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823231128733478.png)
∴
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823231128749539.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823231128780484.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823231128795702.png)
∵
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408232311294041001.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823231129419993.png)
∴
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408232311288111234.png)
即
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823231129731981.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823231128203627.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823231129778789.png)
所以
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823231129794424.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823231129809368.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823231128249652.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823231129872902.png)
于是
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408232311298871219.png)
∴
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408232311299031948.png)
![](http://thumb2018.1010pic.com/images/loading.gif)
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