Euclid:Proposition/X/10/Lemma

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Proposition

In the words of Euclid:

It has been proved in the arithmetical books that similar plane numbers have to one another the ratio which a square number has to a square number,
and that, if two numbers have to one another the ratio which a square number has to a square number, they are similar plane numbers.

And it is manifest from these propositions that numbers which have not to one another the ratio which a square number has to a square number, that is, those which have not their sides proportional, have not to one another the ratio which a square number has to a square number.

(The Elements: Book $\text{X}$: Proposition $10$ : Lemma)


Sources