If A, B, C are angles in a triangle, then cos 2A + cos 2B + cos 2C = -1 – 4 cos A cos B cos C

Описание: If A, B, C are angles in a triangle, then prove that
(i) cos 2A + cos 2B + cos 2C = -1 – 4 cos A cos B cos C
(ii) cos 2A - cos 2B + cos 2C = 1 – 4 sin A cos B sin C
(iii) cos 2A + cos 2B - cos 2C = 1 – 4 sin A sin B cos C
TRIGONOMETRY transformations exercise 6(f).
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