**Equation: **

**This is probably the hardest quadric surface to visualize.
The x-axis in the picture ranges from -10 to 10, the y-axis ranges from
-20 to 20, and the z-axis ranges from -30 to 30. Perhaps the easiest
way to visualize this is to think of the shape of a saddle. In the
xz-plane, we have a parabola. In the yz-plane, we have a parabola
opening in the opposite direction. In planes parallel to the
xy-plane, we see hyperbolas.**