✔ 最佳答案
⒈
cos A + 2cos [(B + C) / 2]
= cos A + 2cos (90° - A / 2)
= cos A + 2sin (A / 2)
= 1 - 2sin² (A / 2) + 2sin (A / 2)
= -2[sin (A / 2) - 1 / 2]² + 3 / 2
當sin(A / 2) = 1 / 2 時,即 A = 60° 時有最大值 3 / 2 #
⒉
sin C = c / 2R
c = 5 , sin B = 4 / 5, R = 13 / 2
sin C = 5 / 13
sin A = sin (B + C) = 33 / 65 #