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Q. The external bisector of an …

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Q. The external bisector of an angle of a triangle divides the opposite side externally in the ratio of the sides containing the angle.
  • 2 answers

Yogita Ingle 5 years, 2 months ago

Exterior angle bisector theorem : The external bisector of an angle of a triangle divides the opposite side externally in the ratio of the sides containing the angle.

Given : A ΔABC, in which AD is the bisector of the exterior ∠A and intersects BC produced in D.

Prove that : BD / CD = AB / AC

Construction : Draw CE || DA meeting AB in E.

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1) CE || DA 1) By construction
2) ∠1 = ∠3 2) Alternate interior angle
3) ∠2 = ∠4 3) Corresponding angle (CE ||DA and BK is a transversal
4) AD is a bisector of ∠A 4) Given
5) ∠1 = ∠2 5) Definition of angle bisector
6) ∠3 = ∠4 6) Transitivity (from 2 and 4)
7) AE = AC 7) If angles are equal then side opposite to them are also equal
8) BD / CD = BA/EA 8) By Basic proportionality theorem(EC ||AD)
9) BD /CD = AB/AE 9) BA = AB and EA = AE
10) BD /CD = AB /AC 10) AE = EC and from(7)

Krishna Kumawat 5 years, 2 months ago

Exterior angle bisector theorem : The external bisector of an angle of a triangle divides the opposite side externally in the ratio of the sides containing the angle. Given : A ΔABC, in which AD is the bisector of the exterior ∠A and intersects BC produced in D.
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