Interactive step by step proof of the van Aubel's
theorem. Instructions: Use the buttons: Next, Previous, First, Last and Go to any
step.

Given an arbitrary planar quadrilateral UTAH, place
a square outwardly on each side, and connect the centers of opposite squares: M,
O, N, and Y. Then van Aubel's theorem states that the two lines MO and NY are of
equal length and cross at a right angle.