bisector | external angle bisector

The exterior angle bisectors, also called the external angle bisectors, of a triangle Δ A B C are the lines bisecting the angles formed by the sides of the triangles and their extensions, as illustrated above. Note that the exterior angle bisectors therefore bisect the supplementary angles of the interior angles, not the entire exterior angles. There are therefore three pairs of oppositely oriented exterior angle bisectors. The exterior angle bisectors intersect pairwise in the so-called excenters J_A, J_B, and J_C. These are the centers of the excircles, i.e., the three circles that are externally tangent to the sides of the triangle (or their extensions).

angle bisector | excenter | excircles | exterior angle | isodynamic points