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