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Homework Help: Cant understand this transition

  1. Jan 14, 2010 #1
    http://i50.tinypic.com/2a6w9qg.jpg

    cant see how they put the only the denominator to be equal the new expression.
    ?
     
  2. jcsd
  3. Jan 14, 2010 #2

    vela

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    They used

    [tex]\frac{1}{1-x}=1+x+x^2+x^3+...[/tex]

    where [itex]x=2z^2/4!+...[/itex]. It looks like they made a sign mistake when they did that, though.
     
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