Let the H be the foot of the perpendicular from D to the line AB produced on the side of A, and let I be the foot of the perpendicular from C to the line AB produced on the side of B. Then the angles HDA and ICB are equal to κ, and HI = .

Also if DH meets the line EC produced on the side of E at J, then DJ = .

From [1] and [2], κ can be eliminated.