H
E
E
L
E
M
E
N
T
S
Let them be produced and meet, in the direction of B, D, at G.
Then, in the triangle GEF,
the exterior angle AEF is equal to the interior and opposite angle EFG: which is impossible. [Prop.16]
Therefore
AB, CD when produced will not meet in the direction B, D.
Similarly it can be proved that neither will they meet towards A, C.
