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Math Help - Trigonometric Equation

  1. #1
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    Trigonometric Equation

    How do you solve the equation 5cos(2x)=2 ?
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  2. #2
    MHF Contributor MarkFL's Avatar
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    Re: Trigonometric Equation

    I would divide through by 5 first. Then I would solve the 2 cases (where k\in\mathbb{Z}):

    a) \cos(2x)=\cos(2x+2k\pi)=\frac{2}{5}

    b) \cos(2x)=\cos(2k\pi-2x)=\frac{2}{5}

    Recall, if \cos(f(x))=a then f(x)=\cos^{-1}(a), where a is an arbitrary constant.
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