A curve whose name means "shell form." Let 
be a curve and 
a fixed point. Let 
and 
be points on a line from 
to 
meeting it at 
, where 
,
with 
a given constant. For example, if 
is a circle
and 
is on 
, then the conchoid is a limaçon,
while in the special case that 
is the diameter of 
, then the conchoid is a cardioid.
The equation for a parametrically represented curve 
with 
is
 |  |  |
(1)
|
 |  |  |
(2)
|
