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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.

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# Confused by Maple

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