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as CF is to FA, so also is BE to EA;
therefore also,
as BE is to EA, so is DG to GA,
and therefore, componendo,
as BA is to AE,
so is DA to AG, [V.18]
and, alternately,
as BA to AD,
so is EA to AG. [V.16]
Therefore
in the parallelograms ABCD, EG,
the sides about the common angle BAD are proportional.
