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14
BOOK I. PROP. XIV. THEOR.
F two straight lines (
and 
) meeting a third straight line (
), at the same point, and at opposite sides of it, make with it adjacent angles (
and 
) equal to two right angles; these straight lines lie in one continuous straight line.
For, if possible let 
, and not , be the continuation of ,
then + 
=
but by the hypothesis + =
∴ = , (ax. 3.); which is absurd (ax. 9.).
∴ , is not the continuation of , and the like may be demonstrated of any other straight line except , ∴ is the continuation of .
Q.E.D.
