Four squares aba1b1, bcb1c1, cdc1d1 and dad1a1 are made out of the four sides AB, BC, CD and Da of the quadrilateral ABCD. The centers of the four squares are m, N, P and Q respectively, connecting MP and NQ. It is proved that MP = NQ and MP is perpendicular to NQ
van Aubel's theorem
I just figured it out,