Question: plot a graph of f and its tangent plane

given this equation: f(x,y) = (xysin(x-y)) / (1+(x^2)+(y^2)) , for (1,1,0) i need to compute the partial derivatives and graph the surface and the tangent plane at the given point. this is as far as i got before getting stuck > f := xysin(x-y)/(1+x^2+y^2); xysin(x - y) ------------ 2 2 1 + x + y > plot3d(f, x = -3 .. 3, y = -3 .. 3, grid = [2, 2]); 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 thanks :)
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