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Math Help - cosine of a double angle

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    cosine of a double angle

    if cos θ=a, express 2θ in terms of a.
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    Quote Originally Posted by blame_canada100 View Post
    if cos θ=a, express 2θ in terms of a.
    Use the identity cos(2\theta) = 2cos^2(\theta) - 1

    (this is derived from the double angle formula for cos and the Pythagorean identity, both of which you'll need to know)

    Knowing cos^2 (\theta) = (cos(\theta))^2 = a^2 you can solve it
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