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Math Help - Where does the sign go?

  1. #1
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    Question Where does the sign go?

    Hello there!
    I am having trouble calculating a very simple derivative, namely the derivative of cos(1/x). I have came to the conclusion, that the result should be -sin(1/x)*1/x^2 = -sin(1/x) / x^2, but the result should be simply sin(1/x)/x^2. I don't understand how of why do we get rid of the minus sign. Thanks in advance!
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  2. #2
    MHF Contributor Siron's Avatar
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    Re: Where does the sign go?

    \frac{d}{dx}\left(\frac{1}{x}\right)=\frac{-1}{x^2}
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  3. #3
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    Re: Where does the sign go?

    Quote Originally Posted by szucslaszlo View Post
    I am having trouble calculating a very simple derivative, namely the derivative of cos(1/x).
    \frac{d}{{dx}}\left[ {\cos \left( {\frac{1}{x}} \right)} \right] = \frac{1}{{x^2 }}\sin \left( {\frac{1}{x}} \right)
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  4. #4
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    Re: Where does the sign go?

    Quote Originally Posted by Siron View Post
    \frac{d}{dx}\left(\frac{1}{x}\right)=\frac{-1}{x^2}
    Oh, I am so embarrased. Thank you!
    Last edited by Ackbeet; December 11th 2011 at 09:15 AM.
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  5. #5
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    Re: Where does the sign go?

    Thanks for both replies!
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