Triangles

 Definition

A triangle is a closed figure consisting of three line segments linked end-to-end. A triangle has three sides and three angles.

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.
Perimeter	The distance around the triangle. The sum of its sides.
Interior angles	The three angles on the inside of the triangle at each vertex.
Exterior angles	The angle between a side of a triangle and the extension of an adjacent side.
Altitude or height	The altitude of a triangle is the perpendicular from one side to the opposite vertex. The three altitudes intersect at a single point, called the orthocenter of the triangle.
Median	The median of a triangle is a line from a vertex to the midpoint of the opposite side.Thethree medians intersect at a single point, called the centroid of the triangle.
Circumcenter	The circumcenter (circuncentro) is the point of intersection of the three perpendicular bisectors of the sides. The circumcenter is also the center of the triangle's circumcircle (circunferencia circunscrita) - the circumference that passes through all three of the triangle's vertex.
Incenter	The point of intersection of the three angles bisector is the incenter (Incentro). The incenter is also the center of the triangle's incircle (circunferencia inscrita)- the largest circle that will fit inside the triangle.

 Also:

1. The shortest side is always opposite the smallest interior angle
2. The longest side is always opposite the largest interior angle

 Properties of all triangles

• The interior angles of a triangle always add up to 180°
• The exterior angles of a triangle always add up to 360°

 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.

Classification of triangles

From: http://www.mathopenref.com/triangle.html