# isosceles trapezoid

An isosceles trapezoid  is a trapezoid  whose legs (http://planetmath.org/Leg) are congruent and that has two congruent angles such that their common side (http://planetmath.org/Side3) is a base of the trapezoid. Thus, in an isosceles trapezoid, any two angles whose common is a base of the trapezoid are congruent.

In Euclidean geometry  , the convention is to state the definition of an isosceles trapezoid without the condition that the legs are congruent, as this fact can be proven in Euclidean geometry from the other requirements. For other geometries, such as hyperbolic geometry and spherical geometry, the condition that the legs are congruent is for the definition of an isosceles trapezoid, as the other requirements do not imply that the legs are congruent.

Below is a picture of an isosceles trapezoid. The common perpendicular to its bases is drawn in cyan.

In some dialects of English (e.g. (http://planetmath.org/Eg) British English), this figure is referred to as an isosceles trapezium. Because of the modifier “isosceles”, no confusion should arise with this usage.

A is a trapezoid that is simultaneously a right trapezoid and an isosceles trapezoid. In Euclidean geometry, such trapezoids are automatically rectangles. In hyperbolic geometry, such trapezoids are automatically Saccheri quadrilaterals. Thus, the phrase “right isosceles trapezoid” occurs rarely.

A 3-sides-equal trapezoid is an isosceles trapezoid having at least three congruent sides. Below is a picture of a 3-sides-equal trapezoid.

In some dialects of English (e.g. British English), this figure is referred to as a 3-sides-equal trapezium. Because of the modifier “3-sides-equal”, no confusion should arise with this usage.

A rare but convenient alternative name for a 3-sides-equal trapezoid is a trisosceles trapezoid; the corresponding name trisosceles trapezium does not seem to be in usage.

 Title isosceles trapezoid Canonical name IsoscelesTrapezoid Date of creation 2013-03-22 17:11:59 Last modified on 2013-03-22 17:11:59 Owner Wkbj79 (1863) Last modified by Wkbj79 (1863) Numerical id 25 Author Wkbj79 (1863) Entry type Definition Classification msc 51-00 Synonym isosceles trapezium Related topic SaccheriQuadrilateral Defines 3-sides-equal trapezoid Defines 3 sides equal trapezoid Defines 3-sides-equal trapezium Defines 3 sides equal trapezium Defines trisosceles trapezoid Defines trisosceles trapezium