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There are some specific elements of the triangle and which are essential to its form, as well as also important to define the main features 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 °. Accordingly, the external angles of a triangle are always supplemental to the inner corner as both combined should be 180°. At the same time, the exterior of each vertex angle is equal to the sum of angles that are not adjacent to it, while the sum of the three external angles should add 360 °.

Triangles can be organized according to its form as well as the type of angles that form inside it. In the first case we have three types of triangles: the equilateral (whose sides are the same and contain the same length), isosceles triangle (which has two sides of the same length and one minor, in addition to which both angles in this smaller segment are the same) and finally the scalenus (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 a triangle (with an angle of 90 °, two sides and a hypotenuse), obtuse-angled triangles (with one angle greater than 90 °), acute-angled triangle (with three angles less than 90 °) and finally, the equiangulo triangle (one who has three internal angles of 90 °).

**Article contributed by the team of collaborators.**