The angle formed by the extension of one side of a triangle and the other adjacent side is called the outer angle of the triangle.

Exterior angle characteristics of triangle:

The vertex of (1) is on a vertex of a triangle, for example, the vertex c of ∠ACD is a vertex of △ABC;

(2) One side of a triangle, such as ∠ACD, and AC is just the side of △ABC;

③ The other side is an extension of one side of the triangle, for example, the CD of ∠ACD is an extension of the BC side of △ABC.

Nature:

(1). The outer angle of a triangle is complementary to its adjacent inner angle.

(2) The outer angle of a triangle is equal to the sum of two non-adjacent inner angles.

(3) The outer angle of a triangle is larger than any inner angle that is not adjacent to it.

④ The sum of the external angles of the triangle is equal to 360.

Let the triangle ABC be the sum of three external angles =(A+B)+(A+C)+(B+C)=360 degrees.

Theorem: An outer angle of a triangle is equal to the sum of two non-adjacent inner angles.

Theorem: The sum of the three internal angles of a triangle is 180 degrees.