The question is from the Blue book. It is from:
It’s test #4, section 3, page 656, #14.
Or from test#3 page 772 #14 (cd book)
Collegeboard say in their answer (this is a geometry question) that:
"Furthermore, angle BEF is an external angle to triangle DBE, but I don’t see how this makes sense. And I am confused what external angle is referring to thanks.
I don’t have the books you mention, but I love teaching geometry. So I’ll give you the exterior angle rules in the hopes that something I say will help you.
An exterior angle is formed by one side of the triangle and the extension of an adjacent side.
It is the supplement of the adjacent interior angle.
But it’s also the sum of the non-adjacent interior angles. So, in triangle ABC, if <A= 40 and < B= 60, then the exterior angle at C will be 100.