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.