trigonometry

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We begin by setting t=sin 18 degrees
we weill think of t an an "unknown" angle and solve to get the exact value.

Use a double angle formula to show that cos 36 degrees=1-2t^2

  • trigonometry -

    cos2x = 1 - 2sin^2 x

    plug in t = sin18 for x and you have your answer,

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