Rolling One Parabola on Another
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The focus of a parabola that rolls around another fixed identical parabola describes a straight line, which is the directrix of the fixed parabola.
Contributed by: Izidor Hafner (August 2012)
Open content licensed under CC BY-NC-SA
 J. Bryant and C. Sangwin, How Round Is Your Circle?: Where Engineering and Mathematics Meet, Princeton, NJ: Princeton University Press, 2008, pp. 44–45.
"Rolling One Parabola on Another"
Wolfram Demonstrations Project
Published: August 15 2012