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as BC is to GH, so is GH to CF,
and,
if three straight lines be proportional, as the first is to the third, so is the figure on the first to the similar and similarly situated figure described on the second, [VI.19, Por.]
therefore,
as BC is to CF, so is the triangle ABC to the triangle KGH.
But
as BC is to CF, so also is the parallelogram BE to the parallelogram EF. [VI.1]
