According to the Consecutive Interior Angles Theorem, if two parallel lines are cut by a transversal, then the pair of consecutive interior angles are supplementary. Therefore, if ∠1 and ∠2 are consecutive interior angles, we know that
m∠1+m∠2=180^(∘).Let's say that ∠1≅ ∠2. This means we can, by the Substitution Property of Equality, rewrite this equation as
m∠1+m∠1=180^(∘).
Let's solve this equation.
