The relation R is defined as l1R l2 ↔ l1 is parallel to l2. To check if R is an equivalence relation, we need to verify the following properties:
Reflexivity: A line is parallel to itself, so l1R l1 holds for all l1, so the relation is reflexive.
Symmetry: If l1 is parallel to l2, then l2 is parallel to l1, so the relation is symmetric.
Transitivity: If l1 is parallel to l2, and l2 is parallel to l3, then l1 is parallel to l3, so the relation is transitive.
Since all three properties hold, the relation R is an equivalence relation.