Prove that cos(2θ)=2cos2(θ)−1\cos(2\theta) = 2\cos^2(\theta) - 1cos(2θ)=2cos2(θ)−1.
Use the Law of Cosines
Use the angle subtraction formula with θ−θ\theta - \thetaθ−θ
Use the Pythagorean identity and the double angle formula for cosine
Both A and B are valid approaches