[math-fun] Dandelin hyperbola proof

Eons ago, trying to prove that locus-of-constant-difference is a conic section, I reinvented Dandelin spheres, with a possibly simpler proof that does not refer to their centers nor radii. gosper.org/hyperb.GIF uses instead the lemma that all tangents to a sphere from a point are equally long. The drawing probably needs stereo vision to be convincing. I'm not sure I realized at the time that the spheres could be unequal. —rwg