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 ,

**References**

Johnson, R. A. *Modern Geometry: An Elementary Treatise on the Geometry of the Triangle and the Circle.* Boston, MA:
Houghton Mifflin, pp. 11-15, 1929.

© 1996-9

1999-05-24