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If A + B + C = 3pi/2, then cos(2A) + cos(2B) + cos(2C) equals:

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

Correct answer: 1 - 4 sinA sinB sinC

Solution

Applying the constraint A + B + C = 3pi/2 reduces the sum to 1 - 4 sinA sinB sinC.

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