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A straight line is at right angles to a plane, when it makes right angles with all the straight lines which meet it and are in the plane.
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Εὐθεῖα πρὸς ἐπίπεδον ὀρθή ἐστιν, ὅταν πρὸς πάσας τὰς ἁπτομένας αὐτῆς εὐθείας καὶ οὔσας ἐν τῷ [ ὑποκειμένῳ ] ἐπιπέδῳ ὀρθὰς ποιῇ γωνίας.
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