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antiparallelogram
In geometry, an antiparallelogram is a quadrilateral in which, like a parallelogram, every two opposite sides have the same length, but in which the two longest sides cross each other instead of being parallel. Antiparallelograms are also called contraparallelograms〔.〕 or crossed parallelograms.〔
A ''crossed parallelograms'' is a special case of a crossed quadrilateral with unequal edges.〔(Quadrilaterals )〕 A special form of the ''crossed parallelogram'' is a crossed rectangle where the short edges are parallel.
==Properties==
Every antiparallelogram has an axis of symmetry through its crossing point. Because of this symmetry, it has two pairs of equal angles as well as two pairs of equal sides.〔 Together with the kites and the isosceles trapezoids, antiparallelograms form one of three basic classes of quadrilaterals with a symmetry axis. The convex hull of an antiparallelogram is an isosceles trapezoid, and every antiparallelogram may be formed from the non-parallel sides (or either pair of parallel sides in case of a rectangle) and diagonals of an isosceles trapezoid.〔.〕
Every antiparallelogram is a cyclic quadrilateral, meaning that its four vertices all lie on a single circle.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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