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Homework Help: Writing Uncertainty in terms of dA, dB, dC

  1. Oct 1, 2012 #1
    1. The problem statement, all variables and given/known data

    Write down formulae for dX in terms of dA, dB, dC if:

    X = [(A) (B^2)] / C

    2. Relevant equations

    I'm using the formula below as this corresponds to this relationship.

    (dX/X)^2 = (dA/A)^2 + (dB/B)^2

    3. The attempt at a solution

    For the first line, not the actual solution, I'm getting either:

    (dX/X)^2 = (dA/A)^2 + (2dB/B)^2 + (dC/C)^2


    (dX/X)^2 = (dA/A)^2 + (dB/B)^2 + (dB/B)^2 + (dC/C)^2

    Then if I follow through on either line, I get a different answer.

    I was wondering which is the correct line to begin with?

    I think it's the second option.

    Thank you.
  2. jcsd
  3. Oct 1, 2012 #2


    User Avatar
    Science Advisor
    Homework Helper

    Hi ZedCar! :smile:

    (try using the X2 button just above the Reply box :wink:)
    No, you should never get any squares of differentials. :redface:

    Start again, with lnX = lnA + … :smile:
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