A∪B=AUC≠B=C cancellation law×。 Example:A={1,2,3},B={3,4,5},C={4,5},B≠C, ButA∪B=AUC ={1,2,3,4,5} Example:A={1,2,3},B={3,4,5},C=3},B≠C, ButA∩B=A∩C={3} A-B=A-C=B=C cancellation law symmetric differenceA∪B=A∪C ⇏ B=C cancellation law 。 Example:A={1,2,3},B={3,4,5},C={4,5}, BC, But A∪B=A∪C={1,2,3,4,5} Example: A={1,2,3},B={3,4,5},C={3},BC, But A∩B=A∩C={3} A-B=A-C ⇏B=C cancellation law :symmetric difference