2023 YTSS P2 Q7

2023 YTSS P2 Q7

Secondary 4
9 marks
  1. By substituting \(3\theta=2\theta+\theta\), show that \(\cos3\theta=4\cos^3\theta-3\cos\theta\).[3]
  2. Hence solve the equation \(4\cos^3\theta-3\cos\theta=\dfrac12\) for \(-180^\circ<\theta<180^\circ\).[6]

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Answer:(a) Proved. (b) \(\theta=\pm20^\circ,\pm100^\circ,\pm140^\circ\).

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