1.r={(a，a)、(b，b)、(c，c)、(d，d)、(a，b)、(b，a)、(c，d)、(d，c)}

2. Because R is symmetric, R-1= R. If the complex relation RR- 1 is needed, then RR-1= R2 = R. 。

3. Because R is reflexive, symmetric and transitive, the reflexive closure, symmetric closure and transitive closure of R are all equal to itself, that is, R (r) = R, S (r) = R, T (r) = R. 。