Formulate and prove a theorem on the sum of the angles of a triangle

Let us prove that the sum of the interior angles of a triangle is 180.

Through vertex B of triangle ABC, draw a straight line DE parallel to the base of AC.

The angle DBE is expanded and is equal to 180.

Angle DBE = ABD + ABC + CBE = 180.

Angle AED = BAC as criss-crossing angles at the intersection of parallel straight lines DE and AC secant AB, similarly, angle BEC = ACB.

Then the angle DBE = 180 = BAC + ABC + ACB, which was required to be proved.