H
E
E
L
E
M
E
N
T
S
the angles BGH, GHD are equal to two right angles,
and
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;
