1
b
: a closed plane curve generated by a point moving in such a way that the sums of its distances from two fixed points is a constant : a plane section of a right circular cone that is a closed curve
- F F' foci
- P P' P'' any point on the curve
- FP + PF' = FP'' + P''F' = FP' + P'F'
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Merriam-Webster unabridged
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