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BOOK I. PROP. XIX. THEOR.
19
F in any triangle
one angle
be greater than another
the side
which is opposite to the greater angle, is greater than the side
opposite the less.
If be not greater than
then must
= or <
.
If =
then
=
(pr. 5.); which is contrary to the hypothesis.
is not less than
for if it were,
<
(pr. 18.) which is contrary to the hypothesis:
Q.E.D.