((3^1/2)b -c )cosa =a cos b , 求cos a
问题描述：
((3^1/2)b -c )cosa =a cos b , 求cos a
答
原题中cos b若改为cosC,则如下：a/sinA =b/sinB =c/sinC =2R（R是△ABC外接圆半径）代入原式化得（√3sinB -sinC)cosA =sinA*cosC即 √3sinB*cosA =sinA*cosC +sinC*cosA=sin(A+C)=sinB 约去sinB得cosA=√3/3...