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Homework Help: Simple Manipultion - what am I missing?

  1. May 20, 2012 #1
    Hi,

    I'm not sure how to go from the top-right to the bottom left.

    It seems like they have differentiated the top and bottom separately... but I think there is some e identity I have forgotten?

    thanks!
     

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  3. May 20, 2012 #2
    It looks like that for k=0 they use l'hopital's rule, because when k=0 you have 0/0 (but maybe this is not the limit). I don't know what is k,N,j, etc.
     
  4. May 22, 2012 #3

    vela

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    Looks kind of sketchy to me. When k=0, they're dividing by zero, so you can't really say the sum is equal to that quotient. It's better if you go back and look at the original series for the k=0 case.
     
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