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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