在△ABC中，角A，B，C所对的边分别为a，b，c，若向量 m→ =（﹣cosB，sinC）， n→ =（﹣cosC，﹣sinB），且 m→⋅n_刷题宝

题干

在△ABC中，角A，B，C所对的边分别为a，b，c，若向量 m→ =（﹣cosB，sinC）， n→ =（﹣cosC，﹣sinB），且 m→⋅n→=

1

2

．

（Ⅰ）求角A的大小；

（Ⅱ）若b+c=4，△ABC的面积 S=3 ，求a的值．

上一题下一题 0.0难度选择题更新时间:2015-03-04 09:50:47

答案(点此获取答案解析）

解：（Ⅰ）∵ m→ =（﹣cosB，sinC），

\vec{n}

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