To find: ∠ B and ∠ C.
Here, Δ ABC is an isosceles triangle since AB = AC
∠ B = ∠ C ……… (i) [Angles opposite to equal sides are equal]
We know, sum of angles in a triangle = 180°
∠ A + ∠ B + ∠ C = 180°
∠ A + ∠ B + ∠ B= 180° (using (i)
120° + 2∠B = 180°
2∠B = 180° – 120°
= 60°
∠B = 30°
Therefore, ∠ B = ∠ C = 30°.