mazin geometry
The figure shows a square ABCD with a line EH through the center O. If AE, BF, CH, and DG are perpendicular to EH, prove that AE2 + BF2 + CH2 + DG2 = AD2.
http://gogeometry.com/school-college/1/p1013-square-vertex-perpendicular-metric-relations-math.htm