The symbol 
denotes the directed angle from 
to 
, which is the signed angle through which 
must be rotated about 
to coincide with 
. Four points 
lie on a circle (i.e., are concyclic) iff 
. It is also true that
 |
Three points 
,
, and 
are collinear iff 
or 
. For any four points, 
, 
,
, and 
,
 |
