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The equation for this strophoid is
x2(a + x)/(a - x).
The curve traced out by points P and P′ which lie on lines through a fixed point A where the midpoint M of PP′ is on a fixed line and the absolute value │PM│ = the absolute value │P′M│ = the absolute value │MO│, where O is the projection of A onto that fixed line.
[Greek strophos, twisted cord (from strephein, to turn; see streb(h)- in Indo-European roots) + -oid.]
American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
(Mathematics) a type of curve on a graph
Collins English Dictionary – Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014