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Shape with three sides / From Wikipedia, the free encyclopedia

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A triangle is a polygon with three edges and three vertices. It is one of the basic shapes in geometry. A triangle with vertices A, B, and C is denoted .

Quick facts: Equilateral triangle, Type, Edges and vertice...
Equilateral triangle
A regular triangle
TypeRegular polygon
Edges and vertices3
Schläfli symbol{3}
Coxeter–Dynkin diagramsCDel_node_1.pngCDel_3.pngCDel_node.png
Symmetry groupDihedral (D3), order 2×3
Internal angle (degrees)60°
PropertiesConvex, cyclic, equilateral, isogonal, isotoxal
Dual polygonSelf
Quick facts: Triangle, Edges and vertices, Schläfli symbol...
A triangle
Edges and vertices3
Schläfli symbol{3} (for equilateral)
Areavarious methods;
see below
Internal angle (degrees)60° (for equilateral)
triangle, tri, three, angle
Triangle = Tri (three) + Angle

In Euclidean geometry, any three points, when non-collinear, determine a unique triangle and simultaneously, a unique plane (i.e. a two-dimensional Euclidean space). In other words, there is only one plane that contains that triangle, and every triangle is contained in some plane. If the entire geometry is only the Euclidean plane, there is only one plane and all triangles are contained in it; however, in higher-dimensional Euclidean spaces, this is no longer true. This article is about triangles in Euclidean geometry, and in particular, the Euclidean plane, except where otherwise noted.