If two parallel lines are cut by a transversal, the consecutive interior angles are supplementary angles.
In the figure above, ∠1\angle 1∠1 and ∠4\angle 4∠4 are supplementary as well as ∠2\angle 2∠2 and ∠3.\angle 3.∠3. m∠1+m∠4=180∘m∠2+m∠3=180∘\begin{aligned} m\angle 1 + m\angle 4 = 180 ^\circ \\ m\angle 2 + m\angle 3 = 180 ^\circ \end{aligned}m∠1+m∠4=180∘m∠2+m∠3=180∘