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An interactive three dimensional plot of a curved surface that this is u-shaped along constant values of one coordinate and increases parabolically as that the other horizontal coordinate is increased. A blue concave up arc is drawn along the surface at the constant x-coordinate 1.5. The slider at the top of the figure changes the location of a highlighted point on the blue curve. The line tangent to the surface in the direction perpendicular to the blue curve at the highlighted point is drawn in black.