Can you quickly spot the mistake in my working ?

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  1. member910132's Avatar
    1 05-05-2012  11:05
    • Banned
    • Posts: 891
    Can you quickly spot the mistake in my working ?
    \displaystyle \int sech x \tanh x\ dx

    So I go:

    \displaystyle \int \frac {\sinh x}{\cosh^2 x}\ dx

    Let u = \cosh x

    So we end up with:

    \displaystyle \int u^{-2}\ du

    Giving us an answer of:

    \displaystyle \frac {-2}{sech^3 x}

    But the MS and Wolfram alpha says the answer is -sech x


    EDIT: How to I get SECH X on latex ? Until then don't answer lol. NVM, instead of writing \sech, I just put sech, I guess there ins't a proper input for sech as there is for the other hyperbolic functions.
    Last edited by member910132; 05-05-2012 at 11:43.
  2. notnek's Avatar
    2 05-05-2012  11:38
    • TSR Demigod
    • Location: Bangkok, Thailand
    Re: Can you quickly spot the mistake in my working ?
    You'll probably kick yourself : what's the integral of u^{-2}? Remember, you're integrating not differentiating.
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  3. member910132's Avatar
    3 05-05-2012  11:43
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    Re: Can you quickly spot the mistake in my working ?
    (Original post by notnek)
    You'll probably kick yourself : what's the integral of u^{-2}? Remember, you're integrating not differentiating.
    Somebody rep this guy for me,

    So we get:

    \displaystyle \int u^{-2}\ du = -sech x
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    1
  4. the bear's Avatar
    4 05-05-2012  15:55
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    • Location: Linton Travel Tavern
    • Posts: 7,204
    Re: Can you quickly spot the mistake in my working ?
    + c
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