Question

If A + B + C = 180°, prove that cosA + cosB + cosC = 1 + 4 cos((B+C)/2) cos((C+A)/2) cos((A+B)/2) = 1 + 4 cos((180 - A)/4) cos((180 - B)/4) cos((180 - C)/4) )

Answer 1

Answer:

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