Conics and Tangent Circles
This sketch is designed to help learners see a connection between the construction of a third circle tangent to two given circles and the conic sections, ellipse and hyperbola. For a more thorough discussion, see Joe Wilson's geometry pages, like http://jwilson.coe.uga.edu/EMAT6680Fa11/Lee/asnmt07hylee/asnmt07hylee.html