Dandelin sphere

Dandelin sphere

[′dänd‚lan‚sfir]
(mathematics)
For a conic that is represented as the intersection of a plane and a circular cone, a sphere that is a tangent to both the plane and the cone.
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He then presents an "ingenious argument" using Dandelin spheres for the ellipse's "shockingly beautiful" characterization in terms of foci--"Is that gorgeous, or what!" (p.