Parallel ⇒ co-interior angles supplementary

Dependencies: Parallel ⇒ corresponding angles equal (Parallel ⇒ corresponding angles equal) and Linear pair sums to 180° (Linear pair sums to 180°).

Statement

Let two lines $\ell_1\parallel\ell_2$ be cut by a third line $t$, meeting at $P\in\ell_1$ and $Q\in\ell_2$. The two interior angles enclosed by $\ell_1$ and $\ell_2$ on the same side of $t$ are called a pair of co-interior angles (also "consecutive interior angles"). Conclusion: each pair of co-interior angles is supplementary, i.e.