Εἰς τὸ λζ΄. Ἀλλὰ τὸ ὑπὸ ΕΘ καὶ τῶν ΕΖ, Γ△, ΚΑ δέδεικται ἴσον
τῷ ὑπὸ τῶν ΕΛ, ΚΘ | Ἐν γὰρ τῷ δευτέρῳ καὶ εἰκοστῷ
θεωρήματι δέδεικται ὅτι αἱ ΕΖ, Γ△, ΚΑ πρὸς τὴν ΘΚ
 τὸν αὐτὸν ἔχουσι λόγον, ὃν ἡ ΛΕ πρὸς ΕΘ· ὥστε τὸ ὑπὸ
τῶν ἄκρων ἴσον ἐστὶ τῷ ὑπὸ τῶν μέσων. Τὸ δὲ ὑπὸ ΕΛ, ΚΘ ἔλασσόν ἐστι τοῦ ἀπὸ ΘΑ | Καὶ
γὰρ τοῦ ὑπὸ ΛΘ, ΘΚ ἴσου ὄντος τοῦ ἀπὸ ΘA, ὥς ἐστι
δῆλον ἐπιζευγνυμένης τῆς ΑΛ καὶ διὰ τοῦτο ὁμοίου
 γινομένου τοῦ ΘΑΚ τριγώνου τῷ ΘΑΛ· ἔσται γὰρ ὡς
ἡ ΛΘ πρὸς ΘΑ, ἡ ΑΘ πρὸς ΟΚ, καὶ τὸ ὑπὸ τῶν ἄκρων
ἴσον τῷ ἀπὸ τῆς μέσης.