Theorem of sum of interior angles of triangle: the sum of three interior angles of triangle is equal to 180.

It is inferred that the two acute angles of 1 right triangle are complementary.

Inference 2 An outer angle of a triangle is equal to the sum of two non-adjacent inner angles.

Inference 3: An outer angle of a triangle is larger than any inner angle that is not adjacent to it.

The sum of the inner angles of a triangle is half of the sum of the outer angles.