Original: The proof depends on an earlier proposition : In a triangle ABC , the exterior angle at C is greater than either of the interior angles A or B/O. This in turn depends on Euclid 's unstated assumption that two straight lines meet in at most one point , a statement not true of elliptic geometry .
Simplified: given a straight line and a point not on this line , you can only draw one straight line through this point that will not meet the other straight line .