In a triangle ABC, AB = AC, ∠BAC = 40° then the external angle at B is :

Answer: Option C

According to question,

Given :
∠A = 40°       AB = AC
∴ ∠B = ∠C
In ΔABC
∠A + ∠B + ∠C = 180°
40° + 2∠B = 180°
2∠B = 180° - 40°
2∠B = 140°
∠B = 70°
∴ External angle at B = 180° - 70° = 110°