Equiangular parallelograms have to one another the ratio compounded of the ratios of their sides.
Let AC and CF be equiangular parallelograms having the angle BCD equal to the angle ECG . I say that the parallelogram AC has to the parallelogram CF the ratio compounded of the ratios of the sides. Let them be placed so that BC is in a straight line with CG . Then DC is also in a straight line with CE . Complete the parallelogram DG .