The measure of an exterior angle of a triangle equals the sum of the measures of the two nonadjacent interior angles

Given

Triangle ABC

Exterior angle of a triangle

  - Two adjacent angles form a straight line (BC)

Prove

Proof

1:  - In a triangle, the sum of the measures of the interior angles is 180°

2: - If the exterior sides of two adjacent angles form a straight line, these angles are supplementary

 - From 1 and 2