A, B, C and D are the points of intersection with the coordinate axes of the lines ax+by=ab and bx+ay=ab then ,

The coordinates of A, B, C and D are A (b, 0), B (0, a), C (n, 0), D (0, b) respectively. Clearly , OA⋅OC=OB⋅OD=abSo, A, B, C, D are concyclic points.