Tag: exterior angle

The exterior angle of a triangle is the angle that forms a linear pair with an interior angle of the the triangle. To draw the exterior angle all you need to do is to extend the side of the triangle. A linear pair is a pair of angle that forms a straight line. In the figure

Theorem. The sum of the interior angles of any triangle is 180°. Here are three proofs for the sum of angles of triangles. Proof 1 uses the fact that the alternate interior angles formed by a transversal with two parallel lines are congruent. Proof 2 uses the exterior angle theorem. Proof 3 uses the idea