Definition:Angle Bisector/External

Definition

 * AngleBisector-Internal-External.png

Let $\angle APB$ be an angle.

Let $BP$ be produced beyond $P$ to $B'$.

The external angle bisector of $\angle APB$ is the straight line which bisects $\angle APB'$.

In the above diagram, $PD$ is the external angle bisector of $\angle APB$.

Thus $\angle APD \cong \angle B'PD$ and $\angle APD + \angle B'PD = \angle APB'$.

Also see

 * Definition:Internal Angle Bisector