The theorem on the sum of the angles of a triangle.

According to the theorem on the sum of the angles of a triangle, it follows that the sum of all three angles of a triangle is 180 degrees.
Proof: let’s say there is a triangle ABC, draw a straight line through the vertex B, which will be parallel to the base of the triangle (AC). On the drawn straight line, mark the point K, provided that the line BC will pass between the point A and K. Since the internal cross-lying angles KBC and ACB formed by the straight line BC with parallel straight lines BK and AC, then the angles KBC and AСI are equal.