H
E
E
L
E
M
E
N
T
S
they are equiangular.
But
in equal and equiangular parallelograms the sides about the equal angles are reciprocally proportional. [VI.14]
Therefore
as AB is to CD,
so is CH to AG.
But
CH is equal to E
and
AG to F;
Therefore
as AB is to CD
so is E to F.
