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If the non-common sides of two adjacent angles form a right angle, then the angles are complementary angles.
As can bee seen, the non-common sides of ∠ 1 and ∠ 2 form a right angle. According to theorem, the following equation holds true.
m∠ 1 + m ∠ 2 = 90^(∘)
Since the measure of ∠ ABC is 90^(∘), by the Angle Addition Postulate, the sum of the measures of ∠ 1 and ∠ 2 is 90^(∘). m∠ 1 + m∠ 2 = m∠ ABC ⇓ m∠ 1 + m∠ 2 = 90^(∘) The above equation implies that ∠ 1 and ∠ 2 are complementary angles.