QL-P11: Nine-point Center QL-Diagonal Triangle QL-P11 is the Nine-point Center of the Diagonal Triangle (QL-Tr1) of the Reference Quadrilateral. Coordinates: 1st CT-coordinate: a2 l2 m n (2 m n SA + n l SB + l m SC - b2 m2 - c2 n2) - m2 n2 (8 Δ2 (l m + l n - m n) - b2 l m SB - c2 l n SC) 1st DT-coordinate: S2 + Sb Sc Properties: QL-P11 lies together with QL-P8, QL-P9 and QL-P10 on QL-L7 (QL-Euler Line). The ratio of distances between QL-P9, QL-P8, QL-P11, QL-P10 in this order is 2 : 1 : 3. QL-P1.QL-P11 // QL-P16.QL-P9. QL-P11 lies on perpendicular bisector F1.F2 (F1, F2 are Foci QL-Co1, QL-Co3).