Given: ΔABC Prove: m∠ZAB = m∠ACB m∠CBA Triangle A C B is shown with its exterior angles. Line A B extends through point X. Line A C extends through point Y. Line C A extends through point X. 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 m∠ZAB m∠CAB = 180° by the . We also know m∠CAB m∠ACB m∠CBA = 180° because . Using substitution, we have m∠ZAB m∠CAB = m∠CAB m∠ACB m∠CBA. Therefore, we conclude m∠ZAB = m∠ACB m∠CBA using the .