题目
题型:不详难度:来源:
![](http://img.shitiku.com.cn/uploads/allimg/20191019/20191019121959-72239.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191019/20191019121959-59754.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191019/20191019121959-29116.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191019/20191019121959-16749.png)
(1)求
![](http://img.shitiku.com.cn/uploads/allimg/20191019/20191019122000-93555.png)
(2)若
![](http://img.shitiku.com.cn/uploads/allimg/20191019/20191019122000-94855.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191019/20191019122000-46395.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191019/20191019122000-13507.png)
答案
![](http://img.shitiku.com.cn/uploads/allimg/20191019/20191019122001-97344.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191019/20191019122001-93418.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191019/20191019122002-28856.png)
解析
试题分析:(1)此类问题需要进行两个统一:1、角的统一2、三角函数的统一,在三角函数的统一过程中往往应用三角的和差倍角公式,因此本题将角A+C转化为B,在应用两角和的余弦公式求出csoA=
![](http://img.shitiku.com.cn/uploads/allimg/20191019/20191019122002-42039.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191019/20191019122001-93418.png)
(2)本题需要搞清投影的概念,向量
![](http://img.shitiku.com.cn/uploads/allimg/20191019/20191019122000-46395.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191019/20191019122000-13507.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191019/20191019122000-46395.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191019/20191019122003-76593.png)
(1)
![](http://img.shitiku.com.cn/uploads/allimg/20191019/20191019122003-78581.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191019/20191019122003-35665.png)
所以
![](http://img.shitiku.com.cn/uploads/allimg/20191019/20191019122003-52974.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191019/20191019122020-89512.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191019/20191019122020-41489.png)
(2)由正弦定理可知
![](http://img.shitiku.com.cn/uploads/allimg/20191019/20191019122020-91667.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191019/20191019122021-31421.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191019/20191019122021-29071.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191019/20191019122000-46395.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191019/20191019122000-13507.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191019/20191019122021-98833.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191019/20191019122001-97344.png)
核心考点
举一反三
(1)求A的大小;(2)若
![](http://img.shitiku.com.cn/uploads/allimg/20191019/20191019121744-58281.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191019/20191019121744-10915.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191019/20191019120712-64152.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191019/20191019120712-62640.png)
A.
![](http://img.shitiku.com.cn/uploads/allimg/20191019/20191019120712-56233.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191019/20191019120713-40868.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191019/20191019120713-73965.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191019/20191019120713-57849.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191019/20191019120703-84990.png)
![](http://img.shitiku.com.cn/uploads/allimg/20191019/20191019120647-61507.png)
A.钝角三角形 | B.直角三角形 | C.锐角三角形 | D.不能确定 |
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