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If A + B + C = 180 deg, then the expression cos(2A) + cos(2B) - cos(2C) is equal to:

1. 1 - 4*sinA*sinB*cosC
2. 1 + 4*sinA*sinB*cosC
3. -1 - 4*cosA*cosB*cosC
4. 4*sinA*sinB*sinC - 1

Correct answer: 1 - 4*sinA*sinB*cosC

Solution

Pairing the first two cosines and substituting C = 180 deg - (A+B) collapses the expression to 1 - 4*sinA*sinB*cosC.

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