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Partial derivative in thermodynamics

  1. May 21, 2012 #1
    So I have a proof and I can't follow the process, I think its because I haven't learned how to do partial derivatives or I've forgotten, anyways can someone tell me if this is a rule in calculus


    I've gotten to

    and the proof I have goes to
    -is this a rule? they were able to switch the denominators of the derivatives which is can see but then they switched the constants aswell If so what is the name so I can read up on it, I can't seem to find it with google
  2. jcsd
  3. May 21, 2012 #2
    That's allowed because U is a function that is in the conditions of Schwarz's theorem (which can be derived from stokes' theorem)
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