Question

in a triangle  ABC ,AB = AC , angle BAC = 40, then external angle at  B is

Answer 1

as the sidess AB=BC then the anglesBAC = angle BCA = 30degree

And external angle = 180 - (sum of the opposite interior angles)
                            = 180 - (30+30)
                            = 180 - 60
                            = 120 degrees

so the external angle B  = 120 degrees

Answer 2

he external b measures = 120°