Given,

∠A =120°

AB = AC

We have to find ∠B and ∠C:

We can observe that Δ ABC is an isosceles triangle since AB = AC

∠B = ∠C (Angle opposite to equal sides are equal) [i]

We know that,

Sum of angles in a triangle is equal to 180^o

∠A + ∠B + ∠C = 180^o

∠A + ∠B + ∠B = 180^o

∠A + 2∠B = 180^o

120^o + 2∠B = 180^o

2∠B = 180^o – 120^o

2∠B = 60^o

∠B = 30^o

∠B = ∠C = 30^o