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An interactive three-dimensional plot of a curved surface that is parabolic in one coordinate direction and cubic in the other. A grid of magenta and yellow traces along the surface are drawn. Two sliders at the top of the figure allow the user to change the location of a highlighted point on the surface. At the highlighted point, a vector is shown in magenta that is tilted to be tangent to the magenta trace through the highlighted point. A yellow vector is also drawn tangent to the yellow trace through the highlighted point. A yellow parallelogram is drawn with sides given by the magenta and yellow vectors.