For let the parallelogram FE be completed.
Since, then, the parallelogram AB is equal to the parallelogram BC,
and FE is another area,
therefore, as AB is to FE, so is BC to FE. [V. 7]
But, as AB is to FE, so is DB to BE, [VI. 1]
and, as BC is to FE, so is GB to BF. [id.]
therefore also, as DB is to BE, so is GB to BF. [V. 11]
Therefore in the parallelograms AB, BC the sides about the equal angles are reciprocally proportional.