This lecture delves into the concept of bisectors in a triangle, exploring the interior and exterior bisectors at point A and their perpendicular relationship. It then discusses the locus of points equidistant from sides AB and AC, which is the union of the two bisectors. The Pythagorean theorem is applied to prove the relationship between the distances of these points. The lecture concludes by demonstrating how the interior bisectors of triangle ABC intersect at the incenter, providing a detailed proof of this geometric property.