题目内容
设e1 ,e2 是空间中两个不共线的向量,已知
=2e1+ke2,
=e1+3e2,
=2e1-e2,且A,B,D三点共线,求k的值
![](http://thumb.1010pic.com/pic1/upload/papers/g02/20121022/20121022095921262365.png)
![](http://thumb.1010pic.com/pic1/upload/papers/g02/20121022/20121022095921374358.png)
![](http://thumb.1010pic.com/pic1/upload/papers/g02/20121022/20121022095921456373.png)
解:∵
=e1+3e2,
=2e1-e2,
=(2e1-e2)-(e1+3e2)=e1-4e2.
∵A,B,D三点共线,
∴
,
∴2e1+ke2 =λ(e1-4e2)=λe1-4λe2,
∵e1,e2是空间两个不共线的向量,
∴![](http://thumb.1010pic.com/pic1/upload/papers/g02/20121022/20121022095921720857.png)
所以k=-8。
![](http://thumb.1010pic.com/pic1/upload/papers/g02/20121022/20121022095921550340.png)
![](http://thumb.1010pic.com/pic1/upload/papers/g02/20121022/20121022095921637363.png)
![](http://thumb.1010pic.com/pic1/upload/papers/g02/20120820/201208201028352241648.png)
∵A,B,D三点共线,
∴
![](http://thumb.1010pic.com/pic1/upload/papers/g02/20120820/201208201028352741301.png)
∴2e1+ke2 =λ(e1-4e2)=λe1-4λe2,
∵e1,e2是空间两个不共线的向量,
∴
![](http://thumb.1010pic.com/pic1/upload/papers/g02/20121022/20121022095921720857.png)
所以k=-8。
![](http://thumb2018.1010pic.com/images/loading.gif)
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