Use the identity for sin(A+B)\sin(A + B)sin(A+B) to express sin2A \sin 2A\,sin2A in terms of sinA \sin A\,sinA and cosA\cos AcosA.
Use the identity for cos(A+B)\cos(A + B)cos(A+B) to express cos2A \cos 2A\,cos2A in terms of sinA \sin A\,sinA and cosA\cos AcosA.
Hence express cos2A \cos 2A\,cos2A in terms of cosA \cos A\,cosA only.
225 exam-style questions on WJEC A Level Maths 3.5 Trigonometry (A-level only), covering 3.5.1 Trigonometry (A-level only), 3.5.2 Trigonometry (A-level only), 3.5.3 Trigonometry (A-level only), 3.5.4 Trigonometry (A-level only), 3.5.5 Trigonometry (A-level only), 3.5.6 Trigonometry (A-level only), 3.5.7 Trigonometry (A-level only), and 3.5.8 Trigonometry (A-level only). Each one has a worked solution and a mark scheme showing where the marks go.