Explore Triangle on MathWorld

A triangle is a three-sided (and three-angled) polygon.

Triangle is an elementary-school-level concept. It is listed in the California State Standards for kindergarten.


Equilateral Triangle: A triangle in which all three sides are of equal length. In such a triangle, the angles are all equal as well.
Isosceles Triangle: An isosceles triangle is a triangle having (at least) two sides of equal length, and therefore also with (at least) two equal angles.
Right Triangle: A right triangle is a triangle that has a right angle. The Pythagorean Theorem is a relationship among the sides of a right triangle.


Polygon: A polygon is a two-dimensional figure that consists of a collection of line segments, joined at their ends.

Classroom Articles on Polygons

  • Decagon
  • Parallelogram
  • Dodecagon
  • Pentagon
  • Hendecagon
  • Quadrilateral
  • Heptagon
  • Rectangle
  • Hexagon
  • Regular Polygon
  • Nonagon
  • Square
  • Octagon
  • Trapezoid

  • Classroom Articles on Geometry (Up to Elementary-School Level)

  • Acute Angle
  • Parallel
  • Angle
  • Perimeter
  • Area
  • Perpendicular
  • Circle
  • Point
  • Cone
  • Prism
  • Cube
  • Pyramid
  • Cylinder
  • Radius
  • Diameter
  • Sphere
  • Geometric Construction
  • Surface Area
  • Obtuse Angle
  • Volume