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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 !

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# Homework Help: Proving Trigonometry equation

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