Triangle -- from Wolfram MathWorld
A triangle is a 3-sided polygon sometimes (but not very commonly) called the trigon. Every triangle has three sides and three angles, some of which may be the same. The sides of a triangle are given special names in the case of a right triangle, with the side opposite the right angle being termed the hypotenuse and the other two sides being known as the legs. All triangles are convex and bicentric. That portion of the plane enclosed by the triangle is called the triangle interior, while the
Lucas Tangents Triangle -- from Wolfram MathWorld
Taylor Center -- from Wolfram MathWorld
The Yanghui Triangle, Part I – The Shaw Show
Integer Triangle -- from Wolfram MathWorld
Octahedron -- from Wolfram MathWorld
Snark -- from Wolfram MathWorld
Feuerbach Triangle -- from Wolfram MathWorld
Triangle Dissection Paradox Paradox, Dissection, Nerd geek
Tessellation -- from Wolfram MathWorld
Antipedal Triangle -- from Wolfram MathWorld
Wolfram MathWorld: The Web's Most Extensive Mathematics Resource
A standard pyramidal Galton board, from Wolfram MathWorld [28
Anticomplementary Triangle -- from Wolfram MathWorld