In a triangle there are two kinds of angles:
The interior angles are formed by two sides.
The exterior angles are formed on one side and its extension.
Properties of the Angles of a Triangle
1. The sum of the interior angles of a triangle equals 180°.
A + B + C = 180º
2. The value of an exterior angle of a triangle equals the sum of the two nonadjacent interior angles.
α = B + C
3. The interior and exterior angles of a triangle at a particular vertex are supplementary, that is to say, they add up 180º.
α = 180º - A