15.解: (1) 由正弦定理可得:
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823202426355832.png)
(其中R为
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823202426261544.png)
外接圆的半径)
因为
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823202426417195.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408232024264331061.png)
------------(2分)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823202426464546.png)
---------------------------------------------------------(3分)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823202426495604.png)
---------------------------------------------------(4分)
又
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823202426527559.png)
-----------------------------------------------------------------(6分)
(2) 在
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823202426261544.png)
中,
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823202426573731.png)
且
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823202426589932.png)
--(7分)
则
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408232024266051762.png)
-----------------------------------(8分)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408232024266201349.png)
------------(9分)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408232024266361429.png)
-----------------(10分)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408232024266671021.png)
---------------------------------------------(11分)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408232024266831282.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823202426417195.png)
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/201408232024263391184.png)
的取值范围为
![](http://thumb.1010pic.com/pic2/upload/papers/20140823/20140823202426729665.png)
-----(12分)