I want to define straight using the definition of having 180 degrees, but how do you explain 180 degrees?
Also what types of symmetry are considered line symmetry?
First a comment. Many who have worked in axiomatic geometry do not like using ‘degrees’ as a measure of angles.
An angle is the union of two rays with a common endpoint; as in
Now a straight angle would be the case in which . are collinear and is between .