BOOK I. PROP. XXVIII. THEOR.
29
F a straight line (
) cutting two other straight lines (
and
), makes the external equal to the internal and opposite angle, at the same side of the cutting line (namely,
=
or
=
), or if it makes the two internal angles at the same side (
and
, or
and
) together equal to two right angles, those two straight lines are parallel.
First, if =
, then
=
(pr. 15.),
Q. E. D.