Complete the parallelogram FE.
V.7
Then since the parallelogram AB equals the parallelogram BC, and FE is another parallelogram, therefore AB is to FE as BC is to FE.
VI.1
V.11
But AB is to FE as DB is to BE, and BC is to FE as BG is to BF. Therefore DB is to BE as BG is to BF.
Therefore in the parallelograms AB and BC the sides about the equal angles are reciprocally proportional.