【简答题】A.Took balanced meals with champagne. B.Ate vegetables and fruit only. C.Refrained from fish or meat. D.Avoided eating rich food.
【判断题】One mother refrained from posting her son's photos online when she considered the matter from her son's perspective. (10.0分)A. √ B. ╳
【简答题】设有X={a,b,c)上关系:R 1 ,R 2 ,R 3 ,R 4 为: (1)R 1 ={(a,b),(a,c),(c,b)); (2)R 2 ={(a,b),(b,c),(c,c)}; (3)R 3 ={(a,b),(b,a),(c,c)}; (4)R 4 ={(a,b),(b,c),(c,a)}. 求它们的传递闭包:t(R 1 ),t(R 2 ),t(R 3 ),t(R 4 ).
【简答题】A.πA,B,C(σA<C(R×S)) B.πR.A,R.B,R.C(σR.A<S.B(R×S)) C.πR.A,S.B,S.C(σR.A<S.C(R×S) D.πS.A,S.B,R.C(σR.A<S.C(R×S))
【简答题】设A={a,b,c},A上的关系R和S定义如下: R={〈a,b〉,〈b,b〉,〈b,c〉},S={〈a,c〉,〈b,c〉,〈c,c〉}, 求关系R∪S,R∩S,R-S,S-R,R S.
【多选题】根据系统和环境的联系方式可将系统划分( )