利用正弦定理：a / sinA = b / sinB = c / sinC = 2R（R为三角形外接圆的半径）所以：sinA = a / 2RsinB = b / 2RsinC = c / 2R代入,得：( b-c )sinA+( c-a )sinB+( a-b )sinC= (b - c)*a / 2R + (c - a)*b / 2R + (...