If two sides of a triangle are equal, the angles opposite to these sides are
equal
Draw a △ABC in which AB =AC,
Draw the bisector AD of ∠A, which meets BC, in D.
Since, AB = AC, ∠BAD = ∠CAD, AD = AD
△ABD ≅ △ACD (by SAS criterion)
Then by c.p.c.t ∠B = ∠C.