An orthodiagonal quadrangle is a quadrangle whose diagonals are perpendicular to each other. If 
,
,
,
and 
are the sides of a quadrangle, then this quadrangle is orthodiagonal iff 
.
If 
is a cyclic orthodiagonal quadrangle, then
the quadrangle formed by the tangents to the circumcircle
through the vertices of 
form a bicentric
quadrilateral 
. The circumcenters
of 
and 
and the point of intersection of the diagonals of 
are collinear.
