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S		 Book I   Proposition 19
For, if not, AC is either equal to AB or less.

Now AC is not equal to AB, for then
    the angle ABC would also have been equal to the angle ACB; [1.5]
but it is not;
therefore AC is not equal to AB.

Neither is AC less than AB, for then
    the angle ABC would also have been less than the angle ACB; [1.18]
but it is not;
therefore AC is not less than AB.
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