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If the non-common sides of two adjacent angles form a right angle, then the angles are complementary angles.
Notice that the non-common sides of ∠ 1 and ∠ 2 form a right angle. By the theorem, the following equation holds.
m∠ 1 + m ∠ 2 = 90^(∘)
Because the angles are adjacent, they fit together to fill ∠ ABC without overlapping. That is exactly the condition the Angle Addition Postulate needs. m∠ 1 + m∠ 2 = m∠ ABC ⇓ m∠ 1 + m∠ 2 = 90^(∘) By the definition of complementary angles, ∠ 1 and ∠ 2 are complementary.