Lemma
MediumConic sections
Gilbert Strang, Calculus, 3rd edition (Wellesley-Cambridge Press; MIT OpenCourseWare RES.18-001) §3.5 Parabolas, Ellipses, and Hyperbolas, #29 · Geometry · Conic sections

The slope of the normal line in Figure A is

This problem is about parabolas, are about ellipses, are about hyperbolas.

The slope of the normal line in Figure A is . The slope of the line from is . By the reflection property,

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Gilbert Strang, Calculus, 3rd edition (Wellesley-Cambridge Press; MIT OpenCourseWare RES.18-001) · §3.5 Parabolas, Ellipses, and Hyperbolas, #29

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