If two angles of a triangle are equal, prove the sides opposite to them are equal. (NCERT Class 9 – Isosceles theorem)

Answer: In △ABC, if ∠B=∠C, draw AD bisecting ∠A. Then triangles ABD and ACD have ∠BAD=∠DAC, ∠B=∠C, and AD common ⇒ triangles congruent (AAS). Thus AB=AC.