b/sinB = a/sinA = b/(2sinA*cosA)所以,cosA = b/2a = 2√6/(2*3) = √6/3因为 b^2 + c^2 - 2bc*cosA = a^2c^2 - 2*2√6*c*√6/3 + (2√6)^2 = 9c^2 - 8c + 15 = 0所以,c = 3 或 c = 5为什么=b/（2sinA*cosA),求解释，谢谢。∠B = 2∠AsinB = sin2A = 2sinA*cosA