Interior Angle Bisector Theorem

Interior Angle Bisector Theorem : The angle bisector of an angle of a triangle divides the opposite side internally in the ratio of the sides containing the angle.

Given : A ΔABC in which AD is the internal bisector of ∠A and meets BC in D.
Prove that : BD / DC = AB / AC
Construction : Draw CE || DA to meet BA produced in E.

Statements	Reasons
1) CE || DA	1) By construction
2) ∠2 = ∠3	2) Alternate interior angles
3) ∠1 = ∠4	3) Corresponding angles
4) AD is the bisector	4) Given
5) ∠1 =∠2	5) Definition of angle bisector
6) ∠3= ∠4	6) From (2) and (3)
7) AE = AC	7) In ΔACE, side opposite to equal angles are equal
8) BD / DC = BA / AE	8) In ΔBCE DA || CE and by BPT theorem
9) BD / DC = AB / AC	9) From (7)

_________________________________________________________________
Examples

1) In the given figure, AD is the bisector of ∠A. If BD = 4 cm,
DC = 3 cm and AB = 6 cm, find AC.

Solution :
In Δ ABC, AD is the bisector of ∠A.

∴ BD / DC = AB / AC ( Angle bisector theorem)

⇒ 4 / 3 = 6 / AC

⇒ 4 AC = 18

⇒ AC = 18 / 4 = 4.5 cm.

---------------------------------------------------------------------------
2) AD is a median of ΔABC. The bisector of ∠ADB and ∠ADC meet AB and AC in E and F respectively.

Given : In ΔABC, AD is the median and DE and DF are angle bisectors of ∠ADB and ∠ADC respectively.

Prove that : EF || BC

Statements	Reasons
1) DE is the angle bisector of ∠ADB	1) Given
2) ∴ AD / DB = AE / EB	2) By interior angle bisector theorem
3) DF is the angle bisector of ∠ADC	3) Given
4) AD / DC = AF / FC	4) By angle bisector theorem
5) AD is the median	5) Given
6) BD = DC	6) By definition of median
7) AD / DB = AF / FC	7) From (6)
8) AE / EB = AF / FC	8) From (2) and (7)

Hence EF || BC

---

Similarity in Triangles

• Similarity in Geometry
• Properties of similar triangles
• Basic Proportionality Theorem(Thales theorem)
• Converse of Basic Proportionality Theorem
• Interior Angle Bisector Theorem
• Exterior Angle Bisector Theorem
• Proofs on Basic Proportionality
• Criteria of Similarity of Triangles
• Geometric Mean of Similar Triangles
• Areas of Two Similar Triangles

Home Page