§7.prop.8ἐὰν ἀριθμὸς ἀριθμοῦ μέρη ᾖ, ἅπερ ἀφαιρεθεὶς ἀφαιρεθέντος, καὶ ὁ λοιπὸς τοῦ λοιποῦ τὰ αὐτὰ μέρη ἔσται, ἅπερ ὁ ὅλος τοῦ ὅλου.
If a number be parts of a number, which an subtracted (number) is of an subtracted (number), then the remainder will also be the same parts of the remainder that the whole is of the whole.
ἀριθμὸς γὰρ ὁ ΑΒ ἀριθμοῦ τοῦ ΓΔ μέρη ἔστω, ἅπερ ἀφαιρεθεὶς ὁ ΑΕ ἀφαιρεθέντος τοῦ ΓΖ· λέγω, ὅτι καὶ λοιπὸς ὁ ΕΒ λοιποῦ τοῦ ΖΔ τὰ αὐτὰ μέρη ἐστίν, ἅπερ ὅλος ὁ ΑΒ ὅλου τοῦ ΓΔ.
κείσθω γὰρ τῷ ΑΒ ἴσος ὁ ΗΘ. ἃ ἄρα μέρη ἐστὶν ὁ ΗΘ τοῦ ΓΔ, τὰ αὐτὰ μέρη ἐστὶ καὶ ὁ ΑΕ τοῦ ΓΖ. διῃρήσθω ὁ μὲν ΗΘ εἰς τὰ τοῦ ΓΔ μέρη τὰ ΗΚ, ΚΘ, ὁ δὲ ΑΕ εἰς τὰ τοῦ ΓΖ μέρη τὰ ΑΛ, ΛΕ·
For let the number AB be parts of the number ΓΔ, which the subtracted AE is of the subtracted ΓΖ; I say that the remainder EB is also the same parts of the remainder ΖΔ that the whole AB is of the whole ΓΔ.
ἔσται δὴ ἴσον τὸ πλῆθος τῶν ΗΚ, ΚΘ τῷ πλήθει τῶν ΑΛ, ΛΕ. καὶ ἐπεί, ὃ μέρος ἐστὶν ὁ ΗΚ τοῦ ΓΔ, τὸ αὐτὸ μέρος ἐστὶ καὶ ὁ ΑΛ τοῦ ΓΖ, μείζων δὲ ὁ ΓΔ τοῦ ΓΖ, μείζων ἄρα καὶ ὁ ΗΚ τοῦ ΑΛ. κείσθω τῷ ΑΛ ἴσος ὁ ΗΜ. ὃ ἄρα μέρος ἐστὶν ὁ ΗΚ τοῦ ΓΔ, τὸ αὐτὸ μέρος ἐστὶ καὶ ὁ ΗΜ τοῦ ΓΖ· καὶ λοιπὸς ἄρα ὁ ΜΚ λοιποῦ τοῦ ΖΔ τὸ αὐτὸ μέρος ἐστίν, ὅπερ ὅλος ὁ ΗΚ ὅλου τοῦ ΓΔ. πάλιν ἐπεί, ὃ μέρος ἐστὶν ὁ ΚΘ τοῦ ΓΔ, τὸ αὐτὸ μέρος ἐστὶ καὶ ὁ ΕΛ τοῦ ΓΖ, μείζων δὲ ὁ ΓΔ τοῦ ΓΖ, μείζων ἄρα καὶ ὁ ΘΚ τοῦ ΕΛ. κείσθω τῷ ΕΛ ἴσος ὁ ΚΝ. ὃ ἄρα μέρος ἐστὶν ὁ ΚΘ τοῦ ΓΔ, τὸ αὐτὸ μέρος ἐστὶ καὶ ὁ ΚΝ τοῦ ΓΖ· καὶ λοιπὸς ἄρα ὁ ΝΘ λοιποῦ τοῦ ΖΔ τὸ αὐτὸ μέρος ἐστίν, ὅπερ ὅλος ὁ ΚΘ ὅλου τοῦ ΓΔ. ἐδείχθη δὲ καὶ λοιπὸς ὁ ΜΚ λοιποῦ τοῦ ΖΔ τὸ αὐτὸ μέρος ὤν, ὅπερ ὅλος ὁ ΗΚ ὅλου τοῦ ΓΔ· καὶ συναμφότερος ἄρα ὁ ΜΚ, ΝΘ τοῦ ΔΖ τὰ αὐτὰ μέρη ἐστίν, ἅπερ ὅλος ὁ ΘΗ ὅλου τοῦ ΓΔ. ἴσος δὲ συναμφότερος μὲν ὁ ΜΚ, ΝΘ τῷ ΕΒ, ὁ δὲ ΘΗ τῷ ΒΑ· καὶ λοιπὸς ἄρα ὁ ΕΒ λοιποῦ τοῦ ΖΔ τὰ αὐτὰ μέρη ἐστίν, ἅπερ ὅλος ὁ ΑΒ ὅλου τοῦ ΓΔ· ὅπερ ἔδει δεῖξαι.
For let ΗΘ be set equal to AB. Therefore, what parts ΗΘ is of ΓΔ, the same parts AE is also of ΓΖ. Let ΗΘ be divided into the parts of ΓΔ, namely ΗΚ, ΚΘ, and ΑΕ into the parts of ΓΖ, namely ΑΛ, ΛΕ; then the multitude of ΗΚ, ΚΘ will be equal to the multitude of ΑΛ, ΛΕ. And since, what part ΗΚ is of ΓΔ, the same part ΑΛ is also of ΓΖ, and ΓΔ is greater than ΓΖ, therefore ΗΚ is also greater than ΑΛ. Let ΗΜ be set equal to ΑΛ. Therefore, what part ΗΚ is of ΓΔ, the same part ΗΜ is also of ΓΖ; therefore the remainder ΜΚ is also the same part of the remainder ΖΔ that the whole ΗΚ is of the whole ΓΔ. Again since, what part ΚΘ is of ΓΔ, the same part ΕΛ is also of ΓΖ, and ΓΔ is greater than ΓΖ, therefore ΘΚ is also greater than ΕΛ. Let ΚΝ be set equal to ΕΛ. Therefore, what part ΚΘ is of ΓΔ, the same part ΚΝ is also of ΓΖ; therefore the remainder ΝΘ is also the same part of the remainder ΖΔ that the whole ΚΘ is of the whole ΓΔ. But the remainder ΜΚ was also proved to be the same part of the remainder ΖΔ that the whole ΗΚ is of the whole ΓΔ; therefore both together ΜΚ, ΝΘ are also the same parts of ΔΖ that the whole ΘΗ is of the whole ΓΔ. And both together ΜΚ, ΝΘ are equal to ΕΒ, and ΘΗ to ΒΑ; therefore the remainder ΕΒ is also the same parts of the remainder ΖΔ that the whole ΑΒ is of the whole ΓΔ; which was to be proved.
§7.prop.9ἐὰν ἀριθμὸς ἀριθμοῦ μέρος ᾖ, καὶ ἕτερος ἑτέρου τὸ αὐτὸ μέρος ᾖ, καὶ ἐναλλάξ, ὃ μέρος ἐστὶν ἢ μέρη ὁ πρῶτος τοῦ τρίτου, τὸ αὐτὸ μέρος ἔσται ἢ τὰ αὐτὰ μέρη καὶ ὁ δεύτερος τοῦ τετάρτου.
If a number be a part of a number, and another be the same part of another, then alternately, what part or parts the first is of the third, the same part or parts the second will also be of the fourth.
ἀριθμὸς γὰρ ὁ Α ἀριθμοῦ τοῦ ΒΓ μέρος ἔστω, καὶ ἕτερος ὁ Δ ἑτέρου τοῦ ΕΖ τὸ αὐτὸ μέρος, ὅπερ ὁ Α τοῦ ΒΓ· λέγω, ὅτι καὶ ἐναλλάξ, ὃ μέρος ἐστὶν ὁ Α τοῦ Δ ἢ μέρη, τὸ αὐτὸ μέρος ἐστὶ καὶ ὁ ΒΓ τοῦ ΕΖ ἢ μέρη.
For let the number A be a part of the number BΓ, and another Δ the same part of another ΕΖ that A is of BΓ; I say that also alternately, what part or parts A is of Δ, the same part or parts BΓ is also of ΕΖ.
ἐπεὶ γὰρ ὃ μέρος ἐστὶν ὁ Α τοῦ ΒΓ, τὸ αὐτὸ μέρος ἐστὶ καὶ ὁ Δ τοῦ ΕΖ, ὅσοι ἄρα εἰσὶν ἐν τῷ ΒΓ ἀριθμοὶ ἴσοι τῷ Α, τοσοῦτοί εἰσι καὶ ἐν τῷ ΕΖ ἴσοι τῷ Δ. διῃρήσθω ὁ μὲν ΒΓ εἰς τοὺς τῷ Α ἴσους τοὺς ΒΗ, ΗΓ, ὁ δὲ ΕΖ εἰς τοὺς τῷ Δ ἴσους τοὺς ΕΘ, ΘΖ· ἔσται δὴ ἴσον τὸ πλῆθος τῶν ΒΗ, ΗΓ τῷ πλήθει τῶν ΕΘ, ΘΖ.
καὶ ἐπεὶ ἴσοι εἰσὶν οἱ ΒΗ, ΗΓ ἀριθμοὶ ἀλλήλοις, εἰσὶ δὲ καὶ οἱ ΕΘ, ΘΖ ἀριθμοὶ ἴσοι ἀλλήλοις, καί ἐστιν ἴσον τὸ πλῆθος τῶν ΒΗ, ΗΓ τῷ πλήθει τῶν ΕΘ, ΘΖ, ὃ ἄρα μέρος ἐστὶν ὁ ΒΗ τοῦ ΕΘ ἢ μέρη, τὸ αὐτὸ μέρος ἐστὶ καὶ ὁ ΗΓ τοῦ ΘΖ ἢ τὰ αὐτὰ μέρη· ὥστε καὶ ὃ μέρος ἐστὶν ὁ ΒΗ τοῦ ΕΘ ἢ μέρη, τὸ αὐτὸ μέρος ἐστὶ καὶ συναμφότερος ὁ ΒΓ συναμφοτέρου τοῦ ΕΖ ἢ τὰ αὐτὰ μέρη.
For since, what part A is of BΓ, the same part Δ is also of ΕΖ, therefore, as many numbers as there are in BΓ equal to A, so many are there also in ΕΖ equal to Δ. Let BΓ be divided into the numbers ΒΗ, ΗΓ equal to A, and ΕΖ into the numbers ΕΘ, ΘΖ equal to Δ; then the multitude of ΒΗ, ΗΓ will be equal to the multitude of ΕΘ, ΘΖ. And since the numbers ΒΗ, ΗΓ are equal to one another, and the numbers ΕΘ, ΘΖ are also equal to one another, and the multitude of ΒΗ, ΗΓ is equal to the multitude of ΕΘ, ΘΖ, therefore, what part or parts ΒΗ is of ΕΘ, the same part or parts ΗΓ is also of ΘΖ; so that also, what part or parts ΒΗ is of ΕΘ, the same part or parts both together BΓ is also of both together ΕΖ.
ἴσος δὲ ὁ μὲν ΒΗ τῷ Α, ὁ δὲ ΕΘ τῷ Δ· ὃ ἄρα μέρος ἐστὶν ὁ Α τοῦ Δ ἢ μέρη, τὸ αὐτὸ μέρος ἐστὶ καὶ ὁ ΒΓ τοῦ ΕΖ ἢ τὰ αὐτὰ μέρη· ὅπερ ἔδει δεῖξαι.
But ΒΗ is equal to A, and ΕΘ to Δ; therefore, what part or parts A is of Δ, the same part or parts BΓ is also of ΕΖ; which was to be proved.