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Confused by Maple

  1. Feb 1, 2008 #1
    Hello,

    I was set the problem to differentiate the following streamfunction in order to find its x and y velocities:

    psi=psinought*tanh(y/d)+mu*exp(-((x-L)^2+y^2)/2*sigma^2)*cos(k(y-vt)).

    Being the lazy sort I am, I called in Maple.

    dpsi/dx worked out perfectly (in that I got the same answer by hand)

    dpsi/dy, however, threw up the following:

    dpsi/dy=(psinought/d)*(1-tanh(y/d)^2)-(mu*y/sigma^2)*exp(...)*cos(...)-mu*exp(...)*sin(...)*D(k)(y-vt)


    the last term D(k)... makes no sense to me. How can differentiating introduce a new variable?

    I know I'm probably being a moron, and I'm certainly no expert in using maple, but anyone who can shine a light in my general direction would be muchly appreciated.
     
  2. jcsd
  3. Feb 1, 2008 #2

    George Jones

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    Gold Member

    Should be k*(y-v*t). Maple has to be told explicitly about multiplication.

    Maple applied the chain rule. It interpreted the brackets as saying that k is a function evaluated at argument y-v*t.
     
  4. Feb 4, 2008 #3
    That makes perfect sense. Thank you.
     
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