Tangent plane in 3D

[Tonig00]

Hello
I would like to draw a tangent plane to a function in 3D APP.

I work in CAS mode in the following example:
For example FZ1(X,Y)=X^1/3 * Y^1/3:
I introduce this equation in "Symbolic View" in de 3D APP. It draws it correctly when I press "Plot".

Then I introduce the plane equation in a point (for example (1,1)):
FZ2(X,Y)=FZ1(1,1)+dFZ1(X,Y)/dX * (X-1) + dFZ1(X,Y)/dY * (Y-1)

When I press Plot it does not draw the plane.

I have tried to derivate it directly dFZ1(X,Y)/dX and it gave me 0,

Could somebody give me an idea to how to trace that plane?

Thanks very much

Toni