Here is a figure illustrating the basic harmonic properties of a complete quadrilateral, resulting from a quadrangle ABCD by extending its sides and drawing its diagonals. {E,F,G} are the diagonal points of the quadrilateral. The properties of this figure are studied in Quadrangle_0.html .
The figure stresses the fact that all quadrangles resulting from ABCD by the suggested way have alternatively the same diagonals forming a harmonic bundle at G (see Harmonic_Bundle.html ). Exercise-1 Find the equations of the various lines with respect to the projective base {A,C,D,G}. Exercise-2 Find the coordinates of the intercepts of the various lines with DC.
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