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Thread: Soling cos equation

  1. #1
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    Soling cos equation

    Given that cos 2@= 1/9, find cos @ in the form √ܽa/ bܾ , where ܽa, ܾb ∈ ℕ

    is there any way to do this without trigometric formulae

    @ stands for theta
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  2. #2
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    Re: Soling cos equation

    $\cos(2 \theta) = \dfrac 1 9$

    $2 \cos^2(\theta) - 1 = \dfrac 1 9$

    $\cos^2(\theta) = \dfrac {10}{18}$

    $\cos(\theta) = \pm \sqrt{\dfrac{10}{18}} = \pm \dfrac{\sqrt{5}}{3}$
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