A geometry book would not be complete if it did not include more than a passing reference to pyramids. The Egyptians have made pyramids famous, and people have traveled many miles to see these impressive structures. Engineers and archeologists are equally impressed with the design and construction of the pyramids.
You probably have an idea of what a typical pyramid looks like. It has a square base and four triangular sides rising up to join at a single point. It might come as a surprise to learn that from a mathematician's perspective, there are many kinds of pyramids. To construct a general pyramid, start with a polygon (it doesn't have to be a square, or even four-sided!) in a plane and a point above the polygon. Connect each vertex of the polygon to this point and voilà! You have a pyramid. The polygon that you started with is called the base of the pyramid, and the point is called the apex of the pyramid. I have drawn a traditional pyramid and a pentagonal pyramid for you in Figure 21.3. Notice that the lateral faces of a pyramid are triangles.
A pyramid is the region formed by joining the vertices of a polygon within a plane to a point outside of the plane.
The base of a pyramid is the planar polygon whose vertices are all joined to the noncoplanar point.
The apex of the pyramid is the noncoplanar point.
Just as it is with prisms, the name of a pyramid is determined by the shape of its base. There are hexagonal pyramids and triangular pyramids. Try to reason out the polygon that forms the base of each of these pyramids. There are also regular pyramids: pyramids whose base is a regular polygon and whose lateral edges are all congruent.
Excerpted from The Complete Idiot's Guide to Geometry © 2004 by Denise Szecsei, Ph.D.. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.