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Math Help - set function = 0

  1. #1
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    set function = 0

    I have to find 7cos(7x)=0 and solve for X. I thought I had it correct, but Mathlab says otherwise.

    Here is my process.

    7cos(7x)=0
    cos(7x)=0 divided the 0 by 7
    7x=cos^{-1}(0) take the Arc-Cos of both sides
    x=\frac{cos^{-1}(0)}{7}

    arc cos rules state the radian must be in [0,pi]
    arc cos(0) = \frac{pi}{2}
    so....

    x=\frac{\frac{pi}{2}}{7}

    x=\frac{pi}{14}

    mathlab says the answer is x=\frac{3pi}{14}
    what gives?
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  2. #2
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    Re: set function = 0

    I forgot to add critical info. The interval is (\frac{pi}{7},\frac{2pi}{7})

    But i still am not sure.
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  3. #3
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    Re: set function = 0

    what is the period of the function cos(7x) (that is, how often do the values repeat)?

    clearly π/14 isn't in the specified interval. what is?
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