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    Double Angle Formula

    How to use formula to express exact values

    The double angle identities take two different formulas
    • sin2θ = 2sinθcosθ
    • cos2θ = cos²θ − sin²θ
      The double angle formulas can be quickly derived from the angle sum formulas
      Here's a reminder of the angle sum formulas:
    • sin(A+B) = sinAcosB + cosAsinB
    • cos(A+B) = cosAcosB − sinAsinB
      • If you let θ = A = B in the double angle identities then you get
    • A + B = 2θ
    • sin(2θ) = sinθcosθ + cosθsinθ
        • sin(2θ) = 2sinθcosθ
    • cos(2θ) = cosθcosθ − sinθsinθ
      • cos(2θ) = cos²θ − sin²θ
    double angle formula | angle sum formula | pythagorean identities
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