In this figure, ABED is a cyclic quad. therefore
( as opposite angles of a cyclic quad. are supplementary) ...
BCFE is also a cyclic Quad. so
for the same reason!
when we add these angles ...
angle bed + angle bed = 190 degree as they form a linear pair..
so equation 1 becomes
but they are co interior angles and if co interior angles sum up to 180 degrees they are parallel... so ab is parallel to cf!