Εἰς τὸ ιϚ΄. Καὶ ἐπεὶ τὸ ὑπὸ τῶν ΒΑ, ΑΗ ἴσον ἐστὶ τῷ ὑπὸ τῶν
Β △Ζ καὶ τῷ ὑπὸ τῆς Α △ καὶ συναμφοτέρου τῆς △Ζ, ΑΗ

 
διὰ τὸ παράλληλον εἶναι τὴν △Ζ τῇ ΑΗ | Ἐπεὶ γὰρ
παράλληλός ἐστιν ἡ △Ζ τῇ ΑΗ, ἔστιν ὡς ἡ ΒΑ πρὸς ΑΗ,
ἡ Β△ πρὸς △Ζ· καὶ διὰ τοῦτο τὸ ὑπὸ τῶν ἄκρων τῶν ΒΑ,
△Ζ ἴσον ἐστὶ τῷ ὑπὸ τῶν μέσων τῶν Β△, ΑΗ. Ἀλλὰ τὸ
 ὑπὸ τῶν ΒΑ, △Ζ ἴσον ἐστὶ τῷ ὑπὸ τῶν Β△, △Ζ καὶ τῷ
ὑπὸ τῶν Α△, △Ζ διὰ τὸ πρῶτον θεώρημα τοῦ β΄ βιβλίου
τῆς Στοιχειώσεως· καὶ τὸ ὑπὸ τῶν Β∠, ΑΗ ἄρα ἴσον
ἐστὶ τῷ τε ὑπὸ Β△, △Ζ καὶ τῷ ὑπὸ Α△, △Ζ. Κοινὸν
προσκείσθω τὸ ὑπὸ △Α, ΑΗ· τὸ ἄρα ὑπὸ Β△, ΑΗ μετὰ
 τοῦ ὑπὸ △Α, ΑΗ, ὅπερ ἐστὶν τὸ ὑπὸ ΒΑ, ΑΗ, ἴσον ἐστὶ
τῷ ὑπὸ Β∠, ∠Ζ καὶ τῷ ὑπὸ Α△, △Ζ καὶ ἔτι τῷ ὑπὸ Α△,
ΑΗ.