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Math Help - Simple Differentiation Problem

  1. #1
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    Simple Differentiation Problem

    Hi,

    f(r) = [V * ln(r/b)] / ln(a/b)

    Differentiating, I get:

    f'(r) = V/[r * ln(a/b)]

    But my notes say:

    f'(r) = V/[r * ln(b/a)]

    The difference is in my answer there is ln(b/a), but in my notes it has ln(a/b). Which is right, and why?

    Thanks

    Edit: The notes then multiply f'(r) by -1, making the answer in the notes correct.
    Last edited by algorithm; June 7th 2010 at 01:59 PM.
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  2. #2
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    The correct one is:

    f'(r) = V/[r * ln(a/b)].
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  3. #3
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    Hi

    Thanks. I just realised I misquoted the notes - there should be a multiplication of f(r) by -1. This is what made the ln(a/b) become ln(b/a).
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