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When two angles form a linear pair, they are supplementary.
Based on the diagram, the following equality holds true.
m∠ A + m∠ B = 180^(∘)
This theorem is also known as the Supplement Theorem.
By the Angle Addition Postulate, we know that the measure of the angle formed by ∠ A and ∠ B is equal to the sum of the measures of ∠ A and ∠ B. However, ∠ A and ∠ B form a straight angle, which has a measure of 180^(∘). This gives us the required equality.
m∠ A + m∠ B = 180^(∘)