AB = BC => Triangle ABC is an Isosceles Triangle Thus, Angle C = Angle A DE = BE => Triangle DEB is an Isosceles Triangle Thus, Angle D = Angle B
Proof that triangle ABC and DEB are similar:
Angle DBE = Angle A (corresponding angle) Angle BDE = Angle C (Both Triangle are Isosceles, if one pair of the angle are equal then the other pair should also be equal)
=> Triangle ABC ~ Triangle DEB (AA)
Therefore, Angle E = Angle B
Find angle B:
Angle C + Angle A + Angle B = 180 40 + 40 + angle B = 180 Angle B = 180 - 80 Angle B = 100 degree
