Chapter 2 - Set Theory - 2.3 Venn Diagrams and Set Operations - Exercise Set 2.3 - Page 83: 147

A complement of a set $A$ is a set containing all elements of the universal set that aren't elements of $A$.

A complement of a set $A$ is a set containing all elements of the universal set that aren't elements of $A$. This makes the following true: $A\cup A'=U$ $A\cap A'=\emptyset$