Triangle; Let A, B, C be three nonlinear points. It is called the set [AB] U [BC] U [AC] which is the combination of line segments [AB], [BC], [AC].

The sum of the dimensions of the interior angles in a triangle is 180 °, the measure of an exterior angle is equal to the sum of the dimensions of two interior angles that are not adjacent to it, and the sum of the dimensions of the exterior angles is 360 °.

Auxiliary elements of triangle;

In a triangle, the segment of an angle between the point where the bisector of the angle intersects the opposite side and the vertex is called a bisector of the triangle.

In a triangle, the line segment that joins the midpoint of a side to the opposite corner, the midpoint of that side of the triangle; length is called the median length of that edge.

The part of the perpendicular drawn from one corner to the opposite edge in a triangle, between the corner and the side, is called the height of this side of the triangle.