Below you can enter a function of two variables.
You can turn on and off the following things:
the graph of f;
the intersection with planes that have a fixed x or y coordinate (i.e. the restriction of f(x,y) to f(a,y) or f(x,b));
the tangents to these curves (the slopes of these lines are the partial derivatives);
the tangent plane (the plane that contains the two tangent lines).
You can move the point you are interested in by either using the sliders, or by dragging the point labelled A.