Projection of Conic Section is Conic Section

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Theorem

Let $K$ be a conic section.

Let $K'$ be a projection of $K$.


Then $K'$ is also a conic section.





Proof




Historical Note

Girard Desargues developed the concept of the conic section in the context of projective geometry.

Hence he was able to demonstrate Projection of Conic Section is Conic Section.

This was widely disregarded at the time, caused in part by the obscure terminology he used, but also because the influence of projective geometry was overshadowed by the contemporary development of analytic geometry.


Sources