Question

If A + B + C =
$\pi$
Prove that
$\cos(a \div 2) - \cos(b \div 2) - \cos(c \div 2)$
=
$- 4 \cos((\pi - a) \div 4) \sin((\pi - b) \div 4) \cos((\pi + c) \div 4)$
​

Answer 1

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