∴∠1=/2 ∠ ABC, ∠ 3 = 1/2 ∠ ADC (definition of angular bisector)
∫≈ 1 =∠2 known)
∴∠ ABC = ∠ ADC (known)
∴∠ 1 = ∠ 3 (equivalent substitution)
∫≈ 1 =∠2 (known)
∴∠2 =∞∠3 (equivalent substitution)
∴(ab)÷(DC) (internal dislocation angles are equal and two straight lines are parallel)
∴∠ A+∠ ADC = 180o, ∠ C+∠ ABC = 180o (two straight lines are parallel and their internal angles are complementary).
∴∠ A =∠ C (complementary angles of equal angles are equal)