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Math Help - bit of help needed on cos-1 differentiation

  1. #1
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    bit of help needed on cos-1 differentiation

    find dy/dx when x=2 if
    y=cos-1 (x)
    i understand that
    -1 du
    squrt 1-u^2 dx
    but now i will get a negative number at the bottom and i cant get a square root of a negative number thank you 4 your time
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  2. #2
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    x = 2 is not in the domain of the inverse trig function y = \cos^{-1}{x}.
    the domain is -1 \leq x \leq 1


    you sure it isn't ...

    y = (\cos{x})^{-1} = \frac{1}{\cos{x}} = \sec{x} ?
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