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•write the formula for cos2A in alternative forms •use the formulae to write trigonometric expressions in diﬀerent forms •use the formulae in the solution of trigonometric equations Contents 1. Introduction 2 2. The double angle formulae for sin2A, cos2A and tan2A 2 3. The formula cos2A = cos2 A −sin2 A 3 4. Finding sin3x in terms of sinx 3 5.