In class today, my instructor went over conic sections and ellipses (and hyperbolas, although that's irrelevant). We pretty much learned the basics - foci, semi-major and semi-minor axes, etc. However, the equation c²= a² − b² where c is the distance from the focus to vertex and b is the distance from the vertex a co-vetex on the minor axis sort of confused me. It sounds a lot like the Pythagorean Theorem. Could someone please explain why this formula is true (graphically)?