ERIC Number: EJ961088
Record Type: Journal
Publication Date: 2011-Sep
Pages: 10
Abstractor: As Provided
ISBN: N/A
ISSN: ISSN-0746-8342
EISSN: N/A
From the Dance of the Foci to a Strophoid
Jobbings, Andrew
College Mathematics Journal, v42 n4 p289-298 Sep 2011
The intersection of a plane and a cone is a conic section and rotating the plane leads to a family of conics. What happens to the foci of these conics as the plane rotates? A classical result gives the locus of the foci as an oblique strophoid when the plane rotates about a tangent to the cone. The analogous curve when the plane intersects a cylinder, in which case all the sections are ellipses, is a right strophoid. This article discusses both results and provides elementary geometric proofs. Rotation about a different axis, such as one meeting the axis of the cone or cylinder, gives a very different curve. We consider how the resulting curve relates to the classical one by analyzing the family of curves obtained as the axis of rotation moves.
Descriptors: Geometric Concepts, Geometry, Mathematics Instruction, Validity, Mathematical Logic, College Mathematics
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Publication Type: Journal Articles; Reports - Descriptive
Education Level: Higher Education
Audience: N/A
Language: English
Sponsor: N/A
Authoring Institution: N/A
Grant or Contract Numbers: N/A