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Ellipse 3D2D. In a cone containing spheres S1 and S2, an ellipse E can be found where S1 and S2 intersect E at one point each. These points form the ellipse focal points F and F'. The distances PF, PF', PQ, and PR in three dimensions are used to prove the sum of PF and PF' is a constant, a two demensional ellipse property.
