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Attack of my horrid math skills, pt. 1

  1. Oct 3, 2007 #1
    I thought I should start numbering these... anyway, here's another brain-fart where I miss something obvious:

    in the book the dude uses:

    [tex]\[
    f(x) = \frac{1}{{30}}\sqrt {a^2 + x^2 } + \frac{1}{{60}}(b - x)
    \]
    [/tex]

    later on he continues, using:

    [tex]\[
    f'(x) = \frac{1}{{30}}\frac{1}{2}(a^2 + x^2 )^{ - 1/2} (2x) - \frac{1}{{60}}
    \]
    [/tex]

    where'd that 2x come from? isn't the derivative of [tex]\[
    \sqrt {a^2 + x^2 }
    \][/tex] just [tex]\[
    \frac{1}{2}(a^2 + x^2 )^{ - 1/2}
    \][/tex]?

    thanks :biggrin:
     
    Last edited: Oct 4, 2007
  2. jcsd
  3. Oct 3, 2007 #2
    Well assuming that you mean x instead of b, that 2x comes from the chain rule.
     
  4. Oct 4, 2007 #3
    yea chain rule but also he seems to have confused x for b in the derivative, should be [tex]\frac{1}{2}(a^2+x^2)^{-1/2}(2x)[/tex]
     
  5. Oct 4, 2007 #4
    blah, I'm an idiot :rofl: . o well, haha thanks.

    and yea those two were typos; I fixed them.
     
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