[alogo] Harmonic properties of quadrilaterals

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 .

[0_0] [0_1] [0_2] [0_3]
[1_0] [1_1] [1_2] [1_3]
[2_0] [2_1] [2_2] [2_3]

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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