The points A and P have the same x-coordinate. The y-coordinate of P is the slope of the line that is tangent to the graph of y = f(x) at A. Move A along the graph, and observe what happens to P.

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Turn off the traces of P by right-clicking on P. You can reset the GeoGebra worksheet by clicking the "reset" button in the upper right corner of the workspace.

Barbara D'Ambrosia, Created with GeoGebra