|Book I Proposition 28|
the angles BGH, GHD are equal to two right angles,
the angles AGH, BGH are also equal to two right angles, [I.13]
the angles AGH, BGH are equal to the angles BGH, GHD.
Let the angle BGH be subtracted from each;
the remaining angle AGH is equal to the remaining angle GHD;
and they are alternate;
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