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Definition: quadrilateral from The Penguin Dictionary of Science

A four-sided ➤polygon.


Summary Article: quadrilateral from The Penguin Dictionary of Mathematics

A plane figure formed by four intersecting lines. A simple quadrilateral is a polygon with four sides. A complete quadrilateral (see diagram) is the figure formed by four lines and their six points of intersection. These points are joined by a further three diagonal lines.

The harmonic property of the complete quadrilateral is that any diagonal line intersects the quadrilateral and the other two diagonal lines in a harmonic range (see cross-ratio). Thus, in the diagram, if DB produced meets GF at Y, then {Y, E; B, D} = -1. Compare quadrangle.

quadrilateral Complete quadrilateral: AC, BD, and FG are diagonal lines.

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