Let’s prove alternate exterior angles ∠1 & ∠8 equal

For parallel lines l and m,

with transversal p,

∠1 = ∠5 (Corresponding angles) …(1)

Also,

∠5 = ∠8 (Vertically opposite angles) …(2)

From (1) & (2)

∠1 = ∠5

Hence,

Alternate Exterior Angles are equal.