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"Proof" in the second volume of the second grade of mathematics
: ∫BE and DF are divided into ∠ABC and ∠ADC (known) respectively.

∴∠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)