9709 P31 - Nov 2022 - Q6 - 7 marks
56
(a) Prove the identity \(\cos 4\theta + 4 \cos 2\theta + 3 \equiv 8 \cos^4 \theta\).
(b) Hence solve the equation \(\cos 4\theta + 4 \cos 2\theta = 4\) for \(0^\circ \leq \theta \leq 180^\circ\).
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