Let L be the set of all straight lines in the Euclidean plane. Two lines l1 and l2 are said to be related by the relation R iff,l1 is parallel to l2. Then, the relation R is
Relation R on the set of all straight lines in the plane is of
parallel line.
A line is parallel to itself. So, R is reflexive.
If l1 is parallel to l2, then l2, is parallel to l1.
R is symmetric relation.
let
l1 is parallel to l2, then l2, is parallel to l3.
Then, l1, is parallel to l3
R is transitive relation.
So, R is equivalence relation.