T H E E L E M E N T S	Book VI   Proposition 31
	And, since     ABC is similar to ABD, therefore     as CB is to BA, so is AB to BD. [VI.Def.1] And,     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.] Therefore,     as CB is to BD,     so is the figure on CB to the similar and similarly described figure on BA.
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