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Coxeter- Exercise 2.2.1(ii)

Prove the following theorem: (ii) The three diagonal lines of a complete quadrilateral are never concurrent. (They are naturally said to form the diagonal triangle of the quadrilateral.) The quadrilateral pqrs has diagonal lines a,b, and c. The lines cannot be concurrent without the lines of the quadrilateral being concurrent which they cannot be by definition of a quadrilateral.