Angle Bisector


Definition : A Line which cuts an angle into two equal halves is called an Angle Bisector.

We already know bisecting means to cutting it into two equal parts. In an angle bisector, it is a line passing through the vertex of the angle that cuts it into two equal smaller angles.


Consider a triangle ABC. Let the angle bisector of angle A intersect side BC at a point D between B and C. The angle bisector theorem states that the ratio of the length of the line segment BD to the length of segment DC is equal to the ratio of the length of side AB to the length of side AC:

And conversely, if a point D on the side BC of triangle ABC divides BC in the same ratio as the sides AB and AC, then AD is the angle bisector of angle  A. The generalized angle bisector theorem states that if D lies on the line BC, then.

This reduces to the previous version if AD is the bisector of  BAC. When D is external to the segment BC, directed line segments and directed angles must be used in the calculation.

The angle bisector theorem is commonly used when the angle bisectors and side lengths are known. It can be used in a calculation or in a proof.

An immediate consequence of the theorem is that the angle bisector of the vertex angle of an isosceles triangle will also bisect the opposite side.