Hence,
R is symmetric Moreover, (T1, T2), (T2, T3) ∈ R ⇒ T1 is  congruent to T2 and T2 is
congruent to T3 ⇒ T1 is congruent to T3 ⇒ (T1, T3) ∈ R Therefore, R is an equivalence
relation Example 3 Let L be the set of all lines in a plane and R be the relation in L defined as
R = {(L1, L2) : L1 is perpendicular to L2}