How do you find the equation of the (sometimes 2 possible) tangent lines between two (or more) circles? like the 2 tangents that cross in the picture on this page: http://mathworld.wolfram.com/Circle-CircleTangents.html. The application for this is for a program that would draw this tangent line and for some reason a couple approaches have failed using derivatives, trying to find the angle with the tangent, etc.. The known information is the coordinates of the centers of the circles and the radii of each circle. I'm not sure what's wrong, but I'm looking for help walking through this kind of simple problem. thanks so much!