Compass and Straightedge Constructions
In the classical geometric constructions, a compass and an unmarked straightedge are the only allowable tools. Any given object might be assumed though, including conic sections. In Conics Apollonius gave solutions to the tangency problems below, using methods very different from my own.
Even with a sharp eye and a steady hand, many of the constructions below are not likely to yield good results on paper. Dynamic geometry software is advisable. The Geometer's Sketchpad is my own weapon of choice. It was used for all of the illustrations in the conics article, including the Java applets. Sketchpad also has several other basic tools, such as the parallel line tool, which can be duplicated with compass and straightedge. In short, if you are not entering any measurements or calculations, you may use these tools with a clear conscience and still call it a compass and straightedge construction. This way the constructions can be executed quickly and with much greater precision than hand work. When version 5 of Sketchpad was released, it debuted a welcome new feature. It is now possible to construct intersection points on loci.
Excepting circles and degenerate cases, it is not possible to construct conics with a compass and straightedge. However, computer technology has opened up a new class of construction. To construct a conic curve, a geometric relationship is constructed. As one point follows a defined circular or linear path, a dependent point can be forced to follow a conic curve. When I first began playing (yes, it is play) with this feature, I soon discovered that it actually is somewhat difficult not to construct a conic.
Last update: January 24, 2012 ... Paul Kunkel email@example.com
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