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S		 Book I   Proposition 21
Again, since in any triangle the exterior angle is greater than the interior and opposite angle, [1.16]
therefore, in the triangle CDE,
    the exterior angle BDC is greater than the angle CED.
(continue)

For the same reason, moreover, in the triangle ABE also,
    the exterior angle CEB is greater than the angle BAC.
(continue)

Since the angle BDC was proved greater than the angle CEB and the angle CEB is greater than the angle BAC;
    therefore the angle BDC is (much) greater than the angle BAC.
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