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Consider △ABC.
The sum of the interior angles of △ABC is 180∘.
m∠A+m∠B+m∠C=180∘
This theorem will be proven using parallel lines and alternate interior angles.
To begin, draw a line, PQ, that passes through B and is parallel to AB. PQ and △ABC create three angles ∠PBA, ∠ABC, and ∠CBQ.