von Aubel's Theorem

$\begin{figure}\begin{center}\BoxedEPSF{vonAubelsTheorem.epsf scaled 800}\end{center}\end{figure}$

Given an arbitrary Quadrilateral, place a Square outwardly on each side, and connect the centers of opposite Squares. Then the two lines are of equal length and cross at a Right Angle.

See also Quadrilateral, Right Angle, Square

References

Kitchen, E. ``Dörrie Tiles and Related Miniatures.'' Math. Mag. 67, 128-130, 1994.