Because of the special assumption on ,it can be written as
In the above is the projection of onto the -plane. Note that, for any , a point provided lies between the surfaces and . Thus the boundary of consists of an upper part the surface a lower part the surface , and possible the lateral part: a cylinder with base and axis parallel to -axis. Thus
): 
----------(71) 
----------(72) 
----------(73) 
,it can be written as 
is the projection of 
onto the 
-plane. Note that, for any 
, a point 
provided 
lies between the surfaces 
and 
. Thus the boundary 
of 
consists of an upper part 
the surface 
a lower part 
the surface 
, and possible the lateral part: 
a cylinder with base 
and axis parallel to 
-axis. Thus
