Trigonometryhard · Past Paper
If A+B+C = 180, then cos 2A + cos 2B + cos 2C = ?
A-1 - 4 cos A cos B cos C
B1 - 4 sin A sin B sin C
C4 cos A cos B cos C
D1 + 4 cos A cos B cos C
✓ Correct Answer: A — -1 - 4 cos A cos B cos C
This is a standard conditional identity for cosines of double angles in a triangle.
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