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Book I   Proposition 18
Then, since
    the angle ADB is an exterior angle of the triangle BCD,
    it is greater than the interior and opposite angle DCB. [1.16]

But
    the angle ADB is equal to the angle ABD, since the side AB is equal to AD;
therefore
    the angle ABD is also greater than the angle ACB;
therefore
    the angle ABC is much greater than the angle ACB.
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