Union of two sets
The union of two sets is
everything in both sets. (with the duplicates removed)
For example if you have the set {3, 4, 5}
and the set {5, 6, 7}, then the union of these two sets is {3, 4, 5, 6, 7}.
The symbol for union is a capital U. So mathematically, the above sentence
would read:
{3, 4, 5} U {5, 6, 7} = {3, 4, 5, 6, 7}
Please note that even though there were 2 fives that the union only contained one.
You can think of the two sets as circles and the union of the two sets as all the elements of both sets, like this:

===>

3 4
5
6 7

<== This kind of a figure, where you show the relationship of sets as circles is called a Venn diagram.

