It is proved that if TM is connected and the intersection point M extends to point P and PB, then ∠ TKM = ∠ ABP (congruence) and ∠ ABP = ∠ NTM (circumferential angle of the same arc), so ∠ TKM = ∠ NTM, and ∝. =TM/MN, so km * Mn = TM * TM = 4.