Triangle
A triangle is a type of polygon.

A triangle. 
Contents
Definition
A triangle is any polygon with three sides, with the smaller angle measures of the intersections of the sides summing to 180 degrees. Triangles exist in Euclidean geometry, and are the simplest possible polygon. In physics, triangles are noted for their durability, since they have only three vertices around with to distort.
Categories
Triangles are split into six categories; three by their angles and three by their side lengths.

An isosceles triangle. 
Equilateral
 Main article: Equilateral triangle
An equilateral triangle has three congruent sides and is also equiangular. Note that all equilateral triangles are similar. All the angles of equilateral triangles are
Isosceles
 Main article: Isosceles triangle
An isosceles triangle has at least two congruent sides (this means that all equilateral triangles are also isosceles), and the two angles opposite the congruent sides are also congruent (this is commonly known as the Hinge theorem).
Scalene
A scalene triangle has no congruent sides, and also no congruent angles (this can be proven by the converse of the Hinge Theorem).
Right

A right triangle. 
 Main article: Right triangle
A right triangle has a right angle, which means the other two angles are complementary. Trigonometry is largely based on right triangles, and the famous Pythagorean Theorem deals with the side lengths of the right triangle. Note that it is impossible to have an equilateral right triangle. It is also impossible to have more than one right angle in a triangle.
Obtuse
An obtuse triangle has an obtuse angle. Note that it is impossible to have an equilateral obtuse triangle. It is also impossible to have more than one obtuse angle in a triangle.
Acute
All the angles of an acute triangle are acute angles.
Related Formulas and Theorems
 The area of any triangle with base and height is . (This can be shown by combining the triangle and a copy of it into a parallelogram).
 The area of any triangle with sides opposite angles is
 The area of any triangle with sides is , where is the semiperimeter (Heron's Formula).
 For a right triangle with legs and hypotenuse , . This is the famous Pythagorean theorem.
 The inradius of a triangle with sides and area is .
 In any triangle, the sum of any two sides is greater than the length of the third side.
 The sum of the interior angles of a triangle is .
 See trigonometric identities for a list of formulae related to trigonometry.
External Links
 Introduction to Geometry by Richard Rusczyk
 Challenging Problems in Geometry  A good book for students who already have a solid handle on elementary geometry.
 Geometry Revisited  A classic.
 Mathematical Problems  Lecture delivered before the International Congress of Mathematicians at Paris in 1900 by David Hilbert.
 Foundations of Geometry by David Hilbert