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Recasting integration variable

  1. Jun 2, 2009 #1
    1. The problem statement, all variables and given/known data

    Recast the integration variable of the following integral to da

    [tex]\tau[/tex]=[tex]\int[/tex]k dr

    k is a positive constant

    2. Relevant equations

    dr= c*dt/a

    3. The attempt at a solution

    I have no idea, I'm sure it's something really simple, but I really just can't think.

    btw, this is part of a bigger problem, that I know how to solve, but I've forgotten how to do this.

    Thanks.
     
    Last edited: Jun 2, 2009
  2. jcsd
  3. Jun 3, 2009 #2

    tiny-tim

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    Welcome to PF!

    Hi maddav! Welcome to PF! :wink:
    Do you mean dt rather than da? :confused:

    If not, can you tell us the bigger problem, so we can see what it all means? :smile:
     
  4. Jun 3, 2009 #3

    HallsofIvy

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    If, as tiny tim suggested, you meant "recast to dt", then just replace the "dr" in the integral with "cdt/a" which you say is equal to it.
     
  5. Jun 3, 2009 #4
    Yeah, the problem was less mathematical than i'd assumed, it's a cosmology question, so it's fairly simple.

    I initially replaced it with dt (prior to it being suggested here), but of course, i'm told that universe is entirely matter and cosmological constant, so I can just rewrite dt as da using the friedmann equation.

    Thanks anyway guys!
     
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