Geometry: Similar Triangles
When you have two congruent triangles, you have triangles that are exact duplicates. If you put one on top of the other (with the correct orientation), you wouldn't be able to see the one on the bottom. If all of the triangles in the world were congruent, all paper airplanes would look alike.
Sometimes your triangles have the same shape but not the same size. You might think that all triangles have the same shape, because they are the same kind of polygon. But that's not what I mean when I am talking about triangles having the same shape. I mean that one triangle is an enlargement of the other (or from the other perspective, one triangle is the reduction of the other). The key to dealing with similar triangles is to put things in proportion.
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.