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AB, BC be equal and equiangular parallelograms having the angles at B equal,
and let
DB,BE be placed in a straight line;
therefore
FB,BG are also in a straight line. [I.14]
I say that, in AB, BC,
the sides about the equal angles are reciprocally proportional,
that is to say, that,
as DB is to BE,
so is GB to BF.
