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Is every one playing a joke on me? PDE solution question

  1. May 19, 2012 #1
    Ok I have read/browse 3 books so far about PDEs.

    They separate variables. Then they suddenly go.

    solution is

    some thing sin
    some thing cos

    some thing -ek
    some thing ek

    Where do sine and cos comes from ?
    How do they know?? where is it explained?
  2. jcsd
  3. May 19, 2012 #2


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    After they separate variables,
    you get several ODEs.
    What are these ODEs?
    Do you recognize the ODEs?
    Is one of them a harmonic oscillator equation?
    (Do you know the Euler identity, relating sin and cos to exp?)
  4. May 19, 2012 #3
    The one I mentioned above was heat equation.

    V2T = 0 (heat equation)

    become this
    X" = -k2X (k power 2)
    Y" = k2y (k power 2)

    Then it say solution of this equation is

    X= sin kx
    cos kx

    Y= e ky (e power ky)
    e -ky (e power -ky)

    And yes I do know Euler's

    ei@ =cos@ + i sin@ ( @ = theta)
  5. May 20, 2012 #4


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    As robphy indicates, it's likely the author simply regards these as well known solutions.
    One way to obtain the result is to multiply both sides of the equation by 2x':
    2 x''.x' = -2k x.x'
    x'^2 = -kx^2 + c
    x' = sqrt(c-kx^2)
    The RHS immediately suggests the substitution x = (sqrt(c/k)) sin @
  6. May 20, 2012 #5


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    You need to study ODEs before you study PDEs. These are simple examples of linear ODEs with constant coefficients. A book on PDEs shouldn't need to explain how to solve them.
  7. May 20, 2012 #6


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    As indicated above, you have to understand how to solve ODEs to understand PDEs. Those two solutions are really the most common form that you should have seen when studying ODEs.

    The only reason I'm adding anything to this is that at first I thought your question was going to be how all of a sudden they threw a hyperbolic sine or cosine at you when you expected a solution of the form you already listed. That's when you'll have to realize that there are many solutions to your ODEs and picking the correct ones can simplify soliving your PDEs significantly.

    Just a heads up so that you don't immediately come running back when they show you different solutions to the same ODEs that you'd been using.
  8. May 20, 2012 #7
    Thank you S-happens, haruspex, robphy.

    I look into it.

    I did read ODEs a little, I think I must have missed stuff.
  9. Nov 9, 2012 #8
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