Proof — Euclid's fifth postulate
Statement
If a straight line falling on two straight lines makes the interior angles on the same side less than two right angles, the two straight lines, if produced indefinitely, meet on that side.
Formula
Parallel postulate (Playfair: through a point not on a line, exactly one parallel exists)
Source
Euclid, Elements, Book I, Postulate 5
Domains
mathematics