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S		 Book I   Proposition 29
    The angle EGB was proved equal to the angle GHD.
    Let the angle BGH be added to each;
therefore
    the angles EGB, BGH are equal to the angles BGH, GHD. [C.N.2]

But
    the angles EGB, BGH are equal to two right angles; [I.13]
therefore
    the angles BGH, GHD are also equal to two right angles.
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