Proposition 3
Construction
To cut off from the longer of two given unequal line segments a part equal to the shorter.
δύο δοθϵισῶν ϵὐθϵιῶν ἀνίσων ἀπὸ τῆς μϵίζονος τῇ ἐλάσσονι ἴσην ϵὐθϵῖαν ἀϕϵλϵῖν.
Given two unequal straight lines, to cut off from the greater a straight line equal to the less.