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Published online by Cambridge University Press: 20 November 2018
If P, Q, R are any three points on a line a and p, q, r are any three lines that pass through a point O and lie in a plane n. which meets the line a in a point A distinct from O, then the three lines l = Qr ⋅ Rq, m = Rp ⋅ P r, n = P q ⋅ Qp lie in a plane (through O).