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Converse Of Isosceles Triangle Theorem

Theorem: Sides opposite to the equal angles in a triangle are equal.

Prerequisites:
AAS congruency (proof)

Proof:

Let ABC be a triangle having \angle B = \angle C. Let us draw AD which bisects the \angle A and meets BC at D.

In \triangle ABD and \triangle ACD,

\qquad\quad\angle B = \angle C\qquad\qquad\qquad\;\:\!\:\!\qquad\text{(given)}\\ \qquad\quad\angle BAD = \angle CAD\qquad\qquad\quad\:\text{(by construction)}
Also, \quad AD = AD\qquad\qquad\qquad\qquad\text{(common)}

Hence, \triangle ABD\cong\triangle ACD by AAS rule.

Thus, by CPCTC,  AB = AC,  i.e,  \triangle ABC is an isosceles triangle.

Q.E.D.


Recommended:
Isosceles Triangle Theorem
Angle Bisector Theorem
Location of centroid

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