§8.prop.18δύο ὁμοίων ἐπιπέδων ἀριθμῶν εἷς μέσος ἀνάλογόν ἐστιν ἀριθμός· καὶ ὁ ἐπίπεδος πρὸς τὸν ἐπίπεδον διπλασίονα λόγον ἔχει ἤπερ ἡ ὁμόλογος πλευρὰ πρὸς τὴν ὁμόλογον πλευράν.
Between two similar plane numbers there is one mean proportional number; and the plane number has to the plane number a ratio duplicate of that which the corresponding side has to the corresponding side.
ἔστωσαν δύο ὅμοιοι ἐπίπεδοι ἀριθμοὶ οἱ Α, Β, καὶ τοῦ μὲν Α πλευραὶ ἔστωσαν οἱ Γ, Δ ἀριθμοί, τοῦ δὲ Β οἱ Ε, Ζ. καὶ ἐπεὶ ὅμοιοι ἐπίπεδοί εἰσιν οἱ ἀνάλογον ἔχοντες τὰς πλευράς, ἔστιν ἄρα ὡς ὁ Γ πρὸς τὸν Δ, οὕτως ὁ Ε πρὸς τὸν Ζ. λέγω οὖν, ὅτι τῶν Α, Β εἷς μέσος ἀνάλογόν ἐστιν ἀριθμός, καὶ ὁ Α πρὸς τὸν Β διπλασίονα λόγον ἔχει ἤπερ ὁ Γ πρὸς τὸν Ε ἢ ὁ Δ πρὸς τὸν Ζ, τουτέστιν ἤπερ ἡ ὁμόλογος πλευρὰ πρὸς τὴν ὁμόλογον.
¦5 Let Α, Β be two similar plane numbers, and let the numbers Γ, Δ be the sides of Α, and Ε, Ζ those of Β. And since similar plane numbers are those which have their sides proportional, therefore, as Γ is to Δ, so is Ε to Ζ. I say then that between Α, Β there is one mean proportional number, and Α has to Β a ratio duplicate of that which Γ has to Ε or Δ to Ζ, that is, of that which the corresponding side has to the corresponding side.
καὶ ἐπεί ἐστιν ὡς ὁ Γ πρὸς τὸν Δ, οὕτως ὁ Ε πρὸς τὸν Ζ, ἐναλλὰξ ἄρα ἐστὶν ὡς ὁ Γ πρὸς τὸν Ε, ὁ Δ πρὸς τὸν Ζ. καὶ ἐπεὶ ἐπίπεδός ἐστιν ὁ Α, πλευραὶ δὲ αὐτοῦ οἱ Γ, Δ, ὁ Δ ἄρα τὸν Γ πολλαπλασιάσας τὸν Α πεποίηκεν.
And since, as Γ is to Δ, so is Ε to Ζ, therefore, alternately, as Γ is to Ε, so is Δ to Ζ. And since Α is a plane number, and Γ, Δ are its sides, therefore Δ by multiplying Γ has made Α.
διὰ τὰ αὐτὰ δὴ καὶ ὁ Ε τὸν Ζ πολλαπλασιάσας τὸν Β πεποίηκεν.
For the same reason also Ε by multiplying Ζ has made Β.
ὁ Δ δὴ τὸν Ε πολλαπλασιάσας τὸν Η ποιείτω.
Now let Δ by multiplying Ε make Η.
καὶ ἐπεὶ ὁ Δ τὸν μὲν Γ πολλαπλασιάσας τὸν Α πεποίηκεν, τὸν δὲ Ε πολλαπλασιάσας τὸν Η πεποίηκεν, ἔστιν ἄρα ὡς ὁ Γ πρὸς τὸν Ε, οὕτως ὁ Α πρὸς τὸν Η. ἀλλʼ ὡς ὁ Γ πρὸς τὸν Ε, ὁ Δ πρὸς τὸν Ζ·
And since Δ by multiplying Γ has made Α, and by multiplying Ε has made Η, therefore, as Γ is to Ε, so is Α to Η. But, as Γ is to Ε, so is Δ to Ζ; therefore also, as Δ is to Ζ, so is Α to Η.
καὶ ὡς ἄρα ὁ Δ πρὸς τὸν Ζ, οὕτως ὁ Α πρὸς τὸν Η. πάλιν, ἐπεὶ ὁ Ε τὸν μὲν Δ πολλαπλασιάσας τὸν Η πεποίηκεν, τὸν δὲ Ζ πολλαπλασιάσας τὸν Β πεποίηκεν, ἔστιν ἄρα ὡς ὁ Δ πρὸς τὸν Ζ, οὕτως ὁ Η πρὸς τὸν Β. ἐδείχθη δὲ καὶ ὡς ὁ Δ πρὸς τὸν Ζ, οὕτως ὁ Α πρὸς τὸν Η·
Again, since Ε by multiplying Δ has made Η, and by multiplying Ζ has made Β, therefore, as Δ is to Ζ, so is Η to Β. But it was also proved that, as Δ is to Ζ, so is Α to Η; therefore also, as Α is to Η, so is Η to Β.
καὶ ὡς ἄρα ὁ Α πρὸς τὸν Η, οὕτως ὁ Η πρὸς τὸν Β. οἱ Α, Η, Β ἄρα ἑξῆς ἀνάλογόν εἰσιν.
Therefore Α, Η, Β are in continued proportion.
τῶν Α, Β ἄρα εἷς μέσος ἀνάλογόν ἐστιν ἀριθμός.
Therefore between Α, Β there is one mean proportional number.
λέγω δή, ὅτι καὶ ὁ Α πρὸς τὸν Β διπλασίονα λόγον ἔχει ἤπερ ἡ ὁμόλογος πλευρὰ πρὸς τὴν ὁμόλογον πλευράν, τουτέστιν ἤπερ ὁ Γ πρὸς τὸν Ε ἢ ὁ Δ πρὸς τὸν Ζ. ἐπεὶ γὰρ οἱ Α, Η, Β ἑξῆς ἀνάλογόν εἰσιν, ὁ Α πρὸς τὸν Β διπλασίονα λόγον ἔχει ἤπερ πρὸς τὸν Η. καί ἐστιν ὡς ὁ Α πρὸς τὸν Η, οὕτως ὅ τε Γ πρὸς τὸν Ε καὶ ὁ Δ πρὸς τὸν Ζ. καὶ ὁ Α ἄρα πρὸς τὸν Β διπλασίονα λόγον ἔχει ἤπερ ὁ Γ πρὸς τὸν Ε ἢ ὁ Δ πρὸς τὸν Ζ· ὅπερ ἔδει δεῖξαι.
I say next that Α also has to Β a ratio duplicate of that which the corresponding side has to the corresponding side, that is, of that which Γ has to Ε or Δ to Ζ. For since Α, Η, Β are in continued proportion, Α has to Β a ratio duplicate of that which it has to Η. And as Α is to Η, so is Γ to Ε and Δ to Ζ. Therefore Α also has to Β a ratio duplicate of that which Γ has to Ε or Δ to Ζ; which was to be proved.