Phenomenically, angle and γωνία have no common origins. But angle actually comes from the Latin angulus (corner) and the word αγκύλος in greek means crooked, curved.
This symbol -> [ ] is also called αγκύλη in greek. because of the two small angles at the edges. So I guess there is some similarity after all. :P
Yes exactly! I would say if you answer this "the triangle has three corners" it is wrong because it implies there are other triangles that do not have three corners. But a triangle by definition has three corners. Only "A triangle has three corners" is correct in my view, and "the triangle has three corners" is wrong.
This clearly illustrates why this construction in general should often be translated into English with a/an. Duolingo moderators should change this across the board.