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Homework Help: Sanity Check - Please

  1. Jun 8, 2009 #1
    1. The problem statement, all variables and given/known data

    Transpose for t,


    I can't get it out of my head but, the soltuion I have come up with doesn't seem right.

    Solution is,

    t=-RC ln(1-x/12)

    Thanks in advance,

    Realted, but not as important as the sanity check. For my own mind, could someone explain to me if this is possible and should/could be carried out. If during my transposing I came up with:

    e^(-t/RC)= (12-x)/12

    Is it possible to invert both sides so that the follwoing is shown


    Again, thanks in advance.
  2. jcsd
  3. Jun 8, 2009 #2


    Staff: Mentor

    For your first problem, that's what I get.
    For the second, yes, it's valid to invert both sides. The idea is that if a/b = c/d, then b/a = d/c, as long as you don't introduce any division by zero in doing this.
  4. Jun 8, 2009 #3

    Thanks Mark. Its just that I tried the invert first off and got completly different colutions once I had substitued values in (why I got confused) .
  5. Jun 8, 2009 #4


    Staff: Mentor

    I'm guessing that you made a mistake when you inverted first. If you have a sum or difference -- a + b -- the reciprocal isn't 1/a + 1/b. It's 1/(a + b). For your problem I'm guessing that you took the reciprocal of 1 - e^(-t/RC) to be 1 - e^(t/RC).
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