You can prove that the quadrilateral ABDF is a parallelogram first, and you need to get ABDF anyway. Then, starting from the fact that two straight lines are parallel and the isosceles angles are equal, you can get ∠ cab = ∠ COD = 90 (the intersection of AC and DF is O), and you can get AC⊥DF, plus you have proved that the quadrilateral ADCF is a parallelogram.