Defintion of A TRIANGLE :Triangle From Latin: tri- "three" , angulus "corner, angle." A closed figure consisting of three line segments linked end-to-end. 

A 3-sided polygon. . ForumlA of a triangle : area : one half times the base length times the height of the triangle 1/2 b * h perimeter: side plus side plus side S+S+S pythagorean theorem : a2+ b2 = c2

Triangle properties Vertex The vertex (plural: vertices) is a corner of the triangle. Every triangle has three vertices. Base The base of a triangle can be any one of the three sides, usually the one drawn at the bottom. You can pick any side you like to be the base. Commonly used as a reference side for calculating the area of the triangle. In an isosceles triangle, the base is usually taken to be the unequal side. Altitude The altitude of a triangle is the perpendicular from the base to the opposite vertex. (The base may need to be extended). Since there are three possible bases, there are also three possible altitudes. The three altitudes intersect at a single point, called the orthocenter of the triangle. See Orthocenter of a Triangle. Exterior angles The angle between a side of a triangle and the extension of an adjacent side. Also: The shortest side is always opposite the smallest interior angle The longest side is always opposite the largest interior angle For more on this see Side / angle relationship in a triangle Terminology

It is usual to name each vertex of a triangle with a single capital (upper-case) letter. The sides can be named with a single small (lower case) letter, and named after the opposite angle. So in the figure on the right, you can see that side b is opposite vertex B, side c is opposite vertex C and so on. 

Alternatively, the side of a triangle can be thought of as a line segment joining two vertices. So then side b would be called

AC. This is the form used on this site because it is consistent across all shapes, not just triangles.

Properties of all triangles These are some well known properties of all triangles. See the section below for a complete list The interior angles of a triangle always add up to 180° The exterior angles of a triangle always add up to 360° Types of Triangle There are seven types of triangle, listed below. Note that a given triangle can be more than one type at the same time. For example, a scalene triangle (no sides the same length) can have one interior angle 90°, making it also a right triangle. This would be called a "right scalene triangle". Isosceles Two sides equal See Isosceles triangle definition Equilateral All sides equal See Equilateral triangle definition Scalene No sides equal See Scalene triangle definition Right Triangle One angle 90°. See Right triangle definition Obtuse One angle greater than 90° See Obtuse triangle definition Acute All angles less than 90° See Acute triangle definition Equiangular All interior angles equal See Equiangular triangle definition

Classifying triangles The seven types of triangle can be classified two ways: by sides and by interior angles. For more on this see Classifying triangles.