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Book III   Proposition 2
Then, since DA is equal to DB,
    the angle DAE is also equal to the angle DBE. [I.5]

And,
    since one side AEB of the triangle DAE is produced,
    the angle DEB is greater than the angle DAE [I.16]

But
    the angle DAE is is equal to the angle DBE;
therefore
    the angle DEB is greater than the angle DBE.
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