I think I've got a proof that angle BDE = 20 degrees. I'll try throwing some diagrams together in Paint Shop Pro. It uses properties of isosceles trapeziums, which may cause some confusion as it seems that Americans call trapeziums trapezoids, and conversely call trapezoids trapeziums!
I've tried to prove it cleanly by angle wrangling, but I'm getting bogged down trying to simplify a bazillion trig expressions and making silly errors..At some point I have to use the angle bisector theorem or the cosine rule...both of which yield ugly expressions that have only worked so far with my calculator... There must be a simple and elegant proof but I'm still searching for it 😳
Originally posted by Agerg I've tried to prove it cleanly by angle wrangling, but I'm getting bogged down trying to simplify a bazillion trig expressions and making silly errors..