The Compendium Geometry is an eBook providing facts, formulas and explanations about geometry. 
Home Analytic Geometry Quadrilateral Quadrilateral  General Definitions  
See also: Triangle  General Definitions, Cyclic Quadrilateral, Kite, Parallelogram and Rhombus, Rectangle and Square, Tangential Quadrilateral, Trapezoid, Varignon Parallelogram, Regular Polygon  
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Quadrilateral  General DefinitionsA quadrilateral is a foursided polygon, which is sometimes also called "quadrangle" or "tetragon". All four vertices of the quadrilateral have to lie on a plane, otherwise it is called a "skew quadrilateral". One can differentiate between three topologies: convex quadrilaterals (left figure), concave quadrilaterals, and crossed quadrilaterals. The sum of the angles in a (noncrossed) quadrilateral is 360°: α + β + γ + δ = 360. The area A of a convex planar quadrilateral is given by


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