Theory of proportions: Book 5 Proposition 9
Translations
Τὰ πρὸς τὸ αὐτὸ τὸν αὐτὸν ἔχοντα λόγον ἴσα ἀλλήλοις ἐστίν: καὶ πρὸς ἃ τὸ αὐτὸ τὸν αὐτὸν ἔχει λόγον, ἐκεῖνα ἴσα ἐστίν. Ἐχέτω γὰρ ἑκάτερον τῶν Α, Β πρὸς τὸ Γ τὸν αὐτὸν λόγον: λέγω, ὅτι ἴσον ἐστὶ τὸ Α τῷ Β. Εἰ γὰρ μή, οὐκ ἂν ἑκάτερον τῶν Α, Β πρὸς τὸ Γ τὸν αὐτὸν εἶχε λόγον: ἔχει δέ: ἴσον ἄρα ἐστὶ τὸ Α τῷ Β. Ἐχέτω δὴ πάλιν τὸ Γ πρὸς ἑκάτερον τῶν Α, Β τὸν αὐτὸν λόγον: λέγω, ὅτι ἴσον ἐστὶ τὸ Α τῷ Β. Εἰ γὰρ μή, οὐκ ἂν τὸ Γ πρὸς ἑκάτερον τῶν Α, Β τὸν αὐτὸν εἶχε λόγον: ἔχει δέ: ἴσον ἄρα ἐστὶ τὸ Α τῷ Β. Τὰ ἄρα πρὸς τὸ αὐτὸ τὸν αὐτὸν ἔχοντα λόγον ἴσα ἀλλήλοις ἐστίν: καὶ πρὸς ἃ τὸ αὐτὸ τὸν αὐτὸν ἔχει λόγον, ἐκεῖνα ἴσα ἐστίν: ὅπερ ἔδει δεῖξαι.
Magnitudes which have the same ratio to the same are equal to one another; and magnitudes to which the same has the same ratio are equal. For let each of the magnitudes A, B have the same ratio to C; I say that A is equal to B. For, otherwise, each of the magnitudes A, B would not have had the same ratio to C; [V. 8] but it has; therefore A is equal to B. Again, let C have the same ratio to each of the magnitudes A, B; I say that A is equal to B. For, otherwise, C would not have had the same ratio to each of the magnitudes A, B; [V. 8] but it has; therefore A is equal to B.