Chapter 5 - Section 5.3 - Double-Angle, Power-Reducing, and Half-Angle Formulas - Exercise Set - Page 682: 90

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One way of deriving the Double-Angle Formula is to use the Addition Formula and some basic identities. For example, $sin2\theta=sin(\theta+\theta)=sin\theta cos\theta + cos\theta sin\theta =2 sin\theta cos\theta$