If two tangents from an external point PPP touch the circle at AAA and BBB, then ∠APB+∠AOB=180∘\angle APB + \angle AOB = 180^\circ∠APB+∠AOB=180∘ (where OOO is the centre). The line OPOPOP bisects both angles ∠APB\angle APB∠APB and ∠AOB\angle AOB∠AOB.