We start with triangle ABC and see that angle ZAB is an exterior angle created by the extension of side AC. Angles ZAB and CAB are a linear pair by definition. We know that mZAB + mCAB = 180 by the definition of complementary angles We also know mCAB + mACB + mCBA = 180 because of the triangle angle sum theorem Using substitution, we have mZAB + mCAB = mCAB + mACB + mCBA. Therefore, we conclude mZAB = mACB + mCBA using the