noun

*P*and

*P*&nbstress; which lie on lines through a fixed point

*A*where the midpoint

*M*of

*PP*&nbstress; is on a fixed line and the absolute value |

*PM*| = the absolute value |

*P*&nbstress;

*M*| = the absolute value |

*MO*|, where

*O*is the projection of

*A*onto that fixed line.

Origin of strophoid

Greek*strophos*

*twisted cord*(

*from*

*strephein*

*to turn*; see

*streb(h)-*in Indo-European roots.)

**-oid**

**strophoid**

The equation for this strophoid is

y2

=

x2(a + x)/(a − x).