I'm new to this forum, anyhow thanks for the admin making such forum, doesn't matter ! I've been trying this calculation for one day still no answer, 1. The problem statement, all variables and given/known data cos (A/2)+cos (B/2)+cos (C/2)=4cos [(B+C)/4]*cos [(C+A)/4]*cos [(A+B)/4] A+B+C = pi 2. Relevant equations 3. The attempt at a solution Cos C+Cos D=2cos[(C+D)/2]*cos[(C-D)/2] with above equation tried no use !