Proposition 38
Theorem
Two triangles (△ABC , △DEF ) on equal bases (ᅵBC ᅵ = ᅵEF ᅵ ) and between the same parallels (infinite line AD ‖ infinite line BC ) have the same area.
τὰ τρίγωνα τὰ ἐπὶ ἴσων βάσϵων ὄντα καὶ ἐν ταῖς αὐταῖς παραλλήλοις ἴσα ἀλλήλοις ἐστίν.
Triangles which are on equal bases and in the same parallels are equal to one another.