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.
[Greekstrophos, twisted cord (fromstrephein, to turn; see streb(h)- in the Appendix of Indo-European roots) + –OID.]
(click for a larger image)strophoid
The equation for this strophoid is
x2(a + x)/(a - x).
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