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Evaluate the expression cos²(a) + cos²(a + 120 deg) + cos²(a - 120 deg).

1. 3/2
2. 1
3. 3/4
4. 1/2

Correct answer: 3/2

Solution

Applying the power-reduction identity gives 3/2 plus half the sum of three cosines equally spaced by 120 deg, which is zero, so the value is 3/2.

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