Pascal’s mystic hexagram
If an hexagon
(not necessarily convex) is inscribed
into a conic (in particular into a circle), then the points of intersections
of opposite sides( with , with and with ) are collinear
. This line is called the Pascal line
of the hexagon.
A very special case happens when the conic degenerates into two lines, however the theorem still holds although this particular case is usually called Pappus theorem.