,
gives us the ellipse with vertices
at 
:
in the planes x = k, y = k, z = k.
Solution: The surface is an elliptic cylinder.
Rewriting the equation
as ,
gives us the ellipse with vertices
at :
Since the equation does not have x as a variable the result is an elliptic
cylinder extended along the x-axis. That is, the surface consist of lines
through and perpendicular to
.
by: gm