[计算题,5分] 设R和S是集合A={a, b, c, d}上的关系,其中R={(a, a),(a, c),(b, c),(c, d)}, S={(a, b),(b, c),(b, d),(d, d)}.计算R•S, R∪S, R-1, S-1•R-1

答案
R•S={(a, b),(c, d)},R∪S={(a, a),(a, b),(a, c),(b, c),(b, d),(c, d),(d, d)}, R-1={(a, a),(c, a),(c, b),(d, c)},S-1•R-1={(b, a),(d, c)}.