From Gogeometry : Using : Congruent triangles ΔABE is congruent to ΔBCF by rotation of pi/2 => <AGF=90° Define H as the middle of AB and K as the middle of AG By construction HD//BF AH=HB=>AK=KG <AGF=90° => <AKD=90° ΔDKG is congruent to ΔDKA (SAS) =>DG=DA Therefore a=b