|Book VI Proposition 31|
ABC is similar to ABD,
CB is to BA, so is AB to BD. [VI.Def.1]
since three straight lines proportional,
as the first is to the third, so is the figure on the first to the similar and similarly described figure on the second. [VI.19, Por.]
as CB is to BD,
so is the figure on CB to the similar and similarly described figure on BA.
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