2. If a thing contains all the elements of each set we want to study, this set can be regarded as a complete set, and u stands for complete set.

3. For example, if U = {0, 1, 2, 3, 4, 5} and A = {0, 2, 4}, then CuA={ 1, 3, 5}→ that is, the complement of A in U.