T
H
E

E
L
E
M
E
N
T
S		 Book VI   Proposition 18
And, since
    the angle CFD is equal to the angle AGB,
and
    the angle DFE to the angle BGH,
therefore
    the whole angle CFE is equal to the whole angle AGH.

For the same reason
    the angle CDE is is also equal to the angle ABH.

And
    the angle at C is also equal to the angle at A,
and
    the angle at E to the angle at H.
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