Planes are parallel to each other if they don't intersect. Planes are
perpendicular to each other if one of the planes contains a line perpendicular
to the other plane. Here are the drawings:
Polyhedra
A polyhedron is a simple closed surface that is the union of
polygons. (Sound familiar? Remember that a polygon
is a simple closed curve that is the union of
segments. Much of three-dimensional geometry is only an extension of two-dimensional, or planar, geometry.) A polyhedron encloses a region in space. It
does so with parts of intersecting planes, each part being a polygon. Each
polygon that composes a side of a polyhedron is called a face
of the
polyhedron. The intersection of two faces is called an edge. The
intersection of three or more faces is called a vertex. Below
are pictured
examples of polyhedra.
The following lessons will discuss certain kinds of polyhedra and their general
forms.