As ABCD is a square, soAB = BC = CD = DA     ...(1)AlsoAK = BL = CM = DN   .....(2)Subtracting (2) from (1)We getAB- AK = BC - BL = CD - CM = DA - DNBK = CL = DM = AN   ...(3)So now we haveAK = BL = CM = DNAN = BK= CL = DMSquaring and addingAK² + AN² = BL² + BK²= CM²  + CL² = DN² + DM²    ...(4)But