Draw triangle ABC. Extend BC to D. Draw CE parallel to BA. ∠ABC = ∠ECD (corresponding angles, BA∥CE with BD transversal). ∠BAC = ∠ACE (alternate angles, BA∥CE with AC transversal). Thus ∠A + ∠B + ∠C = ∠ACE + ∠ECD + ∠ACB = ∠ACD (straight line at C) = 180°.