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A three dimensional plot of a curved surface shown faintly in light gray that this is u-shaped along constant values of one coordinate and increases parabolically as that the other horizontal coordinate is increased. Three mutually perpendicular segments are drawn from the lower corner of the surface and are labled
\(x\) and
\(y\) horizontally and
\(z\) for the vertical segment. Eight red curves are shown along the surface in a direction parallel to the segment labeled
\(x\text{.}\) Eight green curves are shown along the surface in a direction parallel to the segment labeled
\(y\text{.}\) Eight blue curves are shown along the surface with constant steps in the height.