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Hi,
After defining a curve, say r (only tried 2D), you can get x(r) and y(r) getting two functions f(t)=x(r(t)) and g(t)=y(r(t)).
If you start writing derivative(f,t) in Input, the plot is shown, but when you hit Enter, it comes up as Undefined.
The same works in CAS without problems, it seems.
The same problem 
Which version of GeoGebra and OS?
Please post your file
I can confirm Jens observation and it is not helpful to post a file, because the result is an undefined function.
Try: a=curve(3u²,2u*exp(-u), u, -10,10) and define f(t)=x(a) and g(t)=y(a)
Then you can see the described problem.
My system: Windows 10 and GG 5.0533
Greetings
mire2
Thank you, my OS and GG is the same.
Btw. as far as I can see, x(a) and y(a) might be obvious, but it seems it's undocumentet.
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