Proof: ∠3 = <span>∠13 (given) </span>∠15 = <span>∠13 (vertically opposite angles are equal) Since </span>∠13 = ∠3 and ∠13 = ∠15, ∠15 = <span>∠3
Then, a must be parallel to b (transverse line d cuts parallel lines producing converse alternate angles that are equal (ie </span>∠15 = <span>∠3)) </span>
