## GMAT Quantitative Review

**3.0**** ****Math Review**

**3.1 Arithmetic**

**9. Sets**

In mathematics a *set* is a collection of numbers or other objects. The objects are called the *elements* of the set. If *S* is a set having a finite number of elements, then the number of elements is denoted by . Such a set is often defined by listing its elements; for example, is a set with . The order in which the elements are listed in a set does not matter; thus . If all the elements of a set *S* are also elements of a set *T*, then *S* is a *subset* of *T*; for example, is a subset of .

For any two sets *A* and *B*, the *union* of *A* and *B* is the set of all elements that are in *A or* in *B or* in both. The *intersection* of *A* and *B* is the set of all elements that are both in *A and* in *B*. The union is denoted by and the intersection is denoted by . As an example, if and , then and . Two sets that have no elements in common are said to be *disjoint* or *mutually exclusive*.

The relationship between sets is often illustrated with a *Venn diagram* in which sets are represented by regions in a plane. For two sets *S* and *T* that are not disjoint and neither is a subset of the other, the intersection is represented by the shaded region of the diagram below.

This diagram illustrates a fact about any two finite sets *S* and *T*: the number of elements in their union equals the sum of their individual numbers of elements minus the number of elements in their intersection (because the latter are counted twice in the sum); more concisely,

.

This counting method is called the general addition rule for two sets. As a special case, if *S* and *T* are disjoint, then

since .