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A 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. On the lower corner of this curved surface are drawn three mutually perpendicular line segments labeled
\(x\text{,}\) \(y\text{,}\) and
\(z\) signifying origin. In the direction of the segment labeled y from the origin, there is a two dimensional flat grid labeled
\(x\) horizontally and
\(z\) vertically. A branch of a parabola is shown in red from the bottom left of this grid to the top right. This red curve is copied along the curved surface along a direction that is parallel to the segment labeled x (from the origin). In the direction of the segment labeled x from the origin, there is a two dimensional flat grid labeled
\(y\) horizontally and
\(z\) vertically. A parabola is shown in blue pointing downward on this grid with a vertex about half way up the grid. This blue curve is copied along the curved surface along a direction that is parallel to the segment labeled y (from the origin).