Next, let DB be to BE as BG is to BF.
I say that the parallelogram AB equals the parallelogram BC.
VI.1
V.11
Since DB is to BE as BG is to BF, while DB is to BE as the parallelogram AB is to the parallelogram FE, and, BG is to BF as the parallelogram BC is to the parallelogram FE, therefore also AB is to FE as BC is to FE.
V.9
Therefore the parallelogram AB equals the parallelogram BC.
Therefore, in equal and equiangular parallelograms the sides about the equal angles are reciprocally proportional; and equiangular parallelograms in which the sides about the equal angles are reciprocally proportional are equal.