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Thread: derivative of 1/x^(2/5) + 8/x^(5/7)?

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    derivative of 1/x^(2/5) + 8/x^(5/7)?

    I'm completely confused by this
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    Forum Admin topsquark's Avatar
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    Re: derivative of 1/x^(2/5) + 8/x^(5/7)?

    Quote Originally Posted by rhcprule3 View Post
    I'm completely confused by this
    Hint: You know that the derivative of x^n is equal to n \cdot x^{n - 1}...

    -Dan
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    Re: derivative of 1/x^(2/5) + 8/x^(5/7)?

    yes i know, actually got what i think is the derivative and for the problem im working on, I got f'(1) correct but when I plug in 4 for f'(4) it says its incorrect! Error in the system?

    What i got was -2/5(x)^(-3/5) - 5/7(8x)^(-2/7)

    For f'(1) I got -6.114 and I got that correct
    When I got in f' (4) I got -.4395 and it says its incorrect
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  4. #4
    MHF Contributor ebaines's Avatar
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    Re: derivative of 1/x^(2/5) + 8/x^(5/7)?

    Another hint: f(x) can be written as:

    f(x) =  x^{-2/5} + 8x^{-5/7}

    -2/5 -1 is not equal to -3/5. And -5/7 - 1 is not equal to -2/7.
    Last edited by ebaines; Oct 3rd 2013 at 06:11 AM.
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