3D APP & diferentials

[Tonig00]

Hello

I do not find a way to plot the diferential of a function with 2 variables.
The goal is to see the plane tangent in a point to a surface.
For example z= 2*x*y/(x^2+y^2).
I change to capital letters when I put the function in the 3D APP and I asigned to FZ1.
Then I put in the next d(FZ1)/dX.
It plots nothing.

Then I derivate in CAS. It gives me the correct answer. I can plot it. It should be substituted in a point and I suppose then I will get the plane.

But, can you help me with a method to derivate directly in the APP? What I am doing wrong?

I attach some screenshots. I hope it works,
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Thanks to all off you

Toni