Known as one of the simplest and most used geometric figures, the triangle could be described as a figure with three sides that join together forming three vertices or corners (hence its name of tri-angle) and that are also finite from a vertex to the other. By containing the sides in the form of segments not aligned parallel, the triangle is considered a polygon. The name of triangle is applied specifically to triangles that have a flat surface, that is, without volume, since those that do have it then receive variants of the same name. The triangle is represented by the ABC symbology (each letter representing one side).
There are some specific elements of the triangle and that are essential to its shape, as well as important to define the main characteristics of this figure. In this sense, one of the first elements to take into account is the fact that the sum of the internal angles of a triangle always measures 180 °. Therefore, the external angles of a triangle are always supplementary to the internal angle since both combined must form 180 °. At the same time, the exterior angle of each of the vertices is equal to the sum of the angles that are not adjacent to it, while the sum of the three exterior angles must add up to 360 °.
Triangles can be organized according to their shape as well as the type of angles that are formed within it. In the first case we have three types of triangles: the equilateral (whose sides are equal and contain the same length), the triangle isosceles (which has two sides of the same length and a smaller one, in addition to the fact that both angles of this smaller segment are equal) and finally the scalene (which has all sides with different lengths and different angles).
On the other hand, if we take into account the types of angles of a triangle, we can define it as right triangle (with an angle of 90 °, two legs and a hypotenuse), obtuse triangles (with an angle greater than 90 °), acute triangle (with three angles less than 90 °) and finally, the equiangular triangle (the one that has three internal angles of 90 °).
