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Homework Help: Simple derivative question

  1. Oct 22, 2005 #1
    So, the question is: find the gradient of f=ln(r), where r=sqrt(x^2+y^2+z^2)^(1/2).

    For the partial with respect to x, I use the chain rule: df/dr*dr/dx.

    df/dr=1/r
    dr/dx=(1/2)*(2x)=x

    Which would give df/dx = x/r.

    But the book gets x/r^2

    Where does the extra 1/r come from?
     
  2. jcsd
  3. Oct 22, 2005 #2
    This isn't right. Remember to use the chain rule here as well.
     
  4. Oct 22, 2005 #3
    hah. of course. thanks
     
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