Angle bisector
From Conservapedia
An (interior) angle bisector (also called an internal angle bisector) is a line or ray that intersects an angle and evenly divides it into two congruent angles.
The angle bisector theorem concerns the relative lengths of the two segments that a triangle's side is divided into by a line that bisects the opposite angle. It equates the relative lengths of the segments to the relative lengths of the other two sides of the triangle.
