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Comparing real parts in (cos θ + i sin θ)² gives cos 2θ equal to
Acos²θ + sin²θ
B1 − 2 cos²θ
Ccos²θ − sin²θ
D2 sin θ cos θ
Answer & Solution
Correct answer: C. cos²θ − sin²θ
Expanding the square gives cos²θ − sin²θ as the real part, which equals cos 2θ.
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