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In Euclidean plane geometry, a rectangle is any quadrilateral with four right angles. It can also be defined as an equiangular quadrilateral, since equiangular means that all of its angles are equal (360°/4 = 90°). It can also be defined as a parallelogram containing a right angle. A rectangle with four sides of equal length is a square. The term oblong is occasionally used to refer to a non-square rectangle.〔http://www.cimt.plymouth.ac.uk/resources/topics/art002.pdf〕〔(Definition of Oblong ). Mathsisfun.com. Retrieved 2011-11-13. 〕〔 (Oblong – Geometry – Math Dictionary ). Icoachmath.com. Retrieved 2011-11-13. 〕 A rectangle with vertices ''ABCD'' would be denoted as . The word rectangle comes from the Latin ''rectangulus'', which is a combination of ''rectus'' (right) and ''angulus'' (angle). A so-called crossed rectangle is a crossed (self-intersecting) quadrilateral which consists of two opposite sides of a rectangle along with the two diagonals.〔 〕 It is a special case of an antiparallelogram, and its angles are not right angles. Other geometries, such as spherical, elliptic, and hyperbolic, have so-called rectangles with opposite sides equal in length and equal angles that are not right angles. Rectangles are involved in many tiling problems, such as tiling the plane by rectangles or tiling a rectangle by polygons. ==Characterizations== A convex quadrilateral is a rectangle if and only if it is any one of the following:〔Zalman Usiskin and Jennifer Griffin, "The Classification of Quadrilaterals. A Study of Definition", Information Age Publishing, 2008, pp. 34–36 ISBN 1-59311-695-0. 〕〔 〕 * an equiangular quadrilateral * a quadrilateral with four right angles * a parallelogram with at least one right angle * a parallelogram with diagonals of equal length * a parallelogram ''ABCD'' where triangles ''ABD'' and ''DCA'' are congruent * a convex quadrilateral with successive sides ''a'', ''b'', ''c'', ''d'' whose area is .〔 * a convex quadrilateral with successive sides ''a'', ''b'', ''c'', ''d'' whose area is 〔Martin Josefsson, ("Five Proofs of an Area Characterization of Rectangles" ), ''Forum Geometricorum'' 13 (2013) 17–21.〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「rectangle」の詳細全文を読む スポンサード リンク
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