§7.prop.3τριῶν ἀριθμῶν δοθέντων μὴ πρώτων πρὸς ἀλλήλους τὸ μέγιστον αὐτῶν κοινὸν μέτρον εὑρεῖν.
Given three numbers not prime to one another, to find their greatest common measure.
ἔστωσαν οἱ δοθέντες τρεῖς ἀριθμοὶ μὴ πρῶτοι πρὸς ἀλλήλους οἱ α, Β, Γ· δεῖ δὴ τῶν Α, Β, Γ τὸ μέγιστον κοινὸν μέτρον εὑρεῖν.
Let the three given numbers not prime to one another be A, B, Γ; it is required then to find the greatest common measure of A, B, Γ.
εἰλήφθω γὰρ δύο τῶν Α, Β τὸ μέγιστον κοινὸν μέτρον ὁ Δ· ὁ δὴ Δ τὸν Γ ἤτοι μετρεῖ ἢ οὐ μετρεῖ.
For let the greatest common measure of two, A, B, be taken, (namely) Δ; then Δ either measures Γ or does not measure (it).
μετρείτω πρότερον· μετρεῖ δὲ καὶ τοὺς Α, Β· ὁ Δ ἄρα τοὺς Α, Β, Γ μετρεῖ· ὁ Δ ἄρα τῶν Α, Β, Γ κοινὸν μέτρον ἐστίν.
First, let it measure (it); but it also measures A, B; therefore Δ measures A, B, Γ; therefore Δ is a common measure of A, B, Γ.
λέγω δή, ὅτι καὶ μέγιστον.
I say then that it is also the greatest.
εἰ γὰρ μή ἐστιν ὁ Δ τῶν Α, Β, Γ μέγιστον κοινὸν μέτρον, μετρήσει τις τοὺς Α, Β, Γ ἀριθμοὺς ἀριθμὸς μείζων ὢν τοῦ Δ. μετρείτω, καὶ ἔστω ὁ Ε. ἐπεὶ οὖν ὁ Ε τοὺς Α, Β, Γ μετρεῖ, καὶ τοὺς Α, Β ἄρα μετρήσει· καὶ τὸ τῶν Α, Β ἄρα μέγιστον κοινὸν μέτρον μετρήσει.
For, if Δ is not the greatest common measure of A, B, Γ, some number which is greater than Δ will measure the numbers A, B, Γ. Let it measure them, and let it be E. Since, then, E measures A, B, Γ, it will also measure A, B; therefore it will also measure the greatest common measure of A, B.
τὸ δὲ τῶν Α, Β μέγιστον κοινὸν μέτρον ἐστὶν ὁ Δ· ὁ Ε ἄρα τὸν Δ μετρεῖ ὁ μείζων τὸν ἐλάσσονα· ὅπερ ἐστὶν ἀδύνατον.
But the greatest common measure of A, B is Δ; therefore E measures Δ, the greater measuring the less; which is impossible.
οὐκ ἄρα τοὺς Α, Β, Γ ἀριθμοὺς ἀριθμός τις μετρήσει μείζων ὢν τοῦ Δ· ὁ Δ ἄρα τῶν Α, Β, Γ μέγιστόν ἐστι κοινὸν μέτρον.
Therefore no number which is greater than Δ will measure the numbers A, B, Γ; therefore Δ is the greatest common measure of A, B, Γ.
μὴ μετρείτω δὴ ὁ Δ τὸν Γ· λέγω πρῶτον, ὅτι οἱ Γ, Δ οὔκ εἰσι πρῶτοι πρὸς ἀλλήλους.
Next, let Δ not measure Γ; I say first that Γ, Δ are not prime to one another.
ἐπεὶ γὰρ οἱ Α, Β, Γ οὔκ εἰσι πρῶτοι πρὸς ἀλλήλους, μετρήσει τις αὐτοὺς ἀριθμός. ὁ δὴ τοὺς Α, Β, Γ μετρῶν καὶ τοὺς Α, Β μετρήσει, καὶ τὸ τῶν Α, Β μέγιστον κοινὸν μέτρον τὸν Δ μετρήσει·
For, since A, B, Γ are not prime to one another, some number will measure them; and the one measuring A, B, Γ will also measure A, B, and will measure the greatest common measure of A, B, (namely) Δ.
μετρεῖ δὲ καὶ τὸν Γ· τοὺς Δ, Γ ἄρα ἀριθμοὺς ἀριθμός τις μετρήσει· οἱ Δ, Γ ἄρα οὔκ εἰσι πρῶτοι πρὸς ἀλλήλους.
But it also measures Γ; therefore some number will measure the numbers Δ, Γ; therefore Δ, Γ are not prime to one another.
εἰλήφθω οὖν αὐτῶν τὸ μέγιστον κοινὸν μέτρον ὁ Ε. καὶ ἐπεὶ ὁ Ε τὸν Δ μετρεῖ, ὁ δὲ Δ τοὺς Α, Β μετρεῖ, καὶ ὁ Ε ἄρα τοὺς Α, Β μετρεῖ·
Therefore let their greatest common measure be taken, (namely) E. And since E measures Δ, and Δ measures A, B, E also measures A, B.
μετρεῖ δὲ καὶ τὸν Γ· ὁ Ε ἄρα τοὺς Α, Β, Γ μετρεῖ· ὁ Ε ἄρα τῶν Α, Β, Γ κοινόν ἐστι μέτρον.
But it also measures Γ; therefore E measures A, B, Γ; therefore E is a common measure of A, B, Γ.
λέγω δή, ὅτι καὶ μέγιστον.
I say then that it is also the greatest.
εἰ γὰρ μή ἐστιν ὁ Ε τῶν Α, Β, Γ τὸ μέγιστον κοινὸν μέτρον, μετρήσει τις τοὺς Α, Β, Γ ἀριθμοὺς ἀριθμὸς μείζων ὢν τοῦ Ε. μετρείτω, καὶ ἔστω ὁ Ζ. καὶ ἐπεὶ ὁ Ζ τοὺς Α, Β, Γ μετρεῖ, καὶ τοὺς Α, Β μετρεῖ· καὶ τὸ τῶν Α, Β ἄρα μέγιστον κοινὸν μέτρον μετρήσει.
For, if E is not the greatest common measure of A, B, Γ, some number which is greater than E will measure the numbers A, B, Γ. Let it measure them, and let it be Z. And since Z measures A, B, Γ, it also measures A, B; therefore it will also measure the greatest common measure of A, B.
τὸ δὲ τῶν Α, Β μέγιστον κοινὸν μέτρον ἐστὶν ὁ Δ· ὁ Ζ ἄρα τὸν Δ μετρεῖ·
But the greatest common measure of A, B is Δ; therefore Z measures Δ.
μετρεῖ δὲ καὶ τὸν Γ· ὁ Ζ ἄρα τοὺς Δ, Γ μετρεῖ· καὶ τὸ τῶν Δ, Γ ἄρα μέγιστον κοινὸν μέτρον μετρήσει.
But it also measures Γ; therefore Z measures Δ, Γ; therefore it will also measure the greatest common measure of Δ, Γ.
τὸ δὲ τῶν Δ, Γ μέγιστον κοινὸν μέτρον ἐστὶν ὁ Ε· ὁ Ζ ἄρα τὸν Ε μετρεῖ ὁ μείζων τὸν ἐλάσσονα· ὅπερ ἐστὶν ἀδύνατον.
But the greatest common measure of Δ, Γ is E; therefore Z measures E, the greater measuring the less; which is impossible.
οὐκ ἄρα τοὺς Α, Β, Γ ἀριθμοὺς ἀριθμός τις μετρήσει μείζων ὢν τοῦ Ε· ὁ Ε ἄρα τῶν Α, Β, Γ μέγιστόν ἐστι κοινὸν μέτρον· ὅπερ ἔδει δεῖξαι.
Therefore no number which is greater than E will measure the numbers A, B, Γ; therefore E is the greatest common measure of A, B, Γ. (Which was) to be proved.