Question: Plotting 3d graphs


I am trying to plot 3d graphs but I get the following error message written in blue:

Warning, unable to evaluate the function to numeric values in the region; see the plotting command's help page to ensure the calling sequence is correct.


The function im trying to plot is:

f(x,y)=[ xy (x²-y² ] / (x² + y²)=expr


This is what I write in Maple:

plot3d( expr,x=-1..1, y=-1..1)


In advance, thanks.


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