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 AcosA.
Hence, express cos2A \cos 2A\,cos2A in terms of sinA\sin AsinA.
Use the identity for tan(A+B)\tan(A + B)tan(A+B) to express tan2A \tan 2A\,tan2A in terms of tanA\tan AtanA.
137 exam-style questions on Edexcel A Level Maths Trigonometry and Modelling, covering 7.1 Addition Formulae, 7.2 Double Angle Formulae, 7.3 Solving Trigonometric Equations, 7.4 Simplifying a cos x +- b sin x, 7.5 Proving Trigonometric Identities, and 7.6 Modelling with Trigonometric Functions. Each one has a worked solution and a mark scheme showing where the marks go.