Venn Euler diagrams are useful for a visual representation of sets. In OBMEP 2022 a question appeared, using this theory. In this post, we will describe operations with these diagrams. Union of sets Union of sets is one of the most used operations on sets. It will be the starting point for operations with sets, defined by: Definition (Union of sets) Given two sets $A$ and $B,$ the union of these sets is formed by all the elements that belong to $A$ or $B$ ($A\cup B$). In algebraic terms, we can write: $$A\cup B = \\{x;~x\in A ~\text{ ou } x\in B\\}$$