Sec. 5.2 Parallelograms 63
asAC=BDwould implyB∈AC. SimilarlyC∈IR(|BAD)soTis on[B,D]. Thus
[A,C]∩[B,D]=0 so as in 2.4.4 a convex quadrilateral/ [A,B,C,D]can be defined, and
in this case it is called aparallelogram. The terminology of 2.4.4 then applies.
A