本试题主要是考查了向量共线以及向量的数量积性质的综合运用。
(1)因为4a-c=(4sin
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823233128516712.png)
4sin
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823233128531420.png)
∵(4a-c)∥b,∴4sin
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823233128406292.png)
cos
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823233128562399.png)
.∴sin
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823233128594496.png)
,进而解得。
(2)a+b=(sin
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823233128625432.png)
cos
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823233128640366.png)
|a+b|
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823233128656982.png)
,转换为三角函数,利用三角函数的性质得到范围。
(1)4a-c=(4sin
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823233128516712.png)
4sin
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823233128531420.png)
……………2分
∵(4a-c)∥b,∴4sin
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823233128406292.png)
cos
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823233128562399.png)
.∴sin
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823233128594496.png)
. ……………4分
∵
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823233128781720.png)
∴
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823233128796517.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823233128828309.png)
,
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823233128828309.png)
). ∴
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823233128874597.png)
或
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823233128906493.png)
即
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823233128438554.png)
或
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823233128469493.png)
. ………6分
(2)a+b=(sin
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823233128625432.png)
cos
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823233128640366.png)
|a+b|
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823233128656982.png)
…………………2分
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408232331290621628.png)
…………………3分
∵
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823233129077664.png)
∴
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823233129093759.png)
. ∴sin
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823233129108949.png)
.
∴
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823233129140386.png)
sin
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823233129155920.png)
. ……………5分
∴|a+b|
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823233128484584.png)
. …………………6分