# Complement of an intersection

The complement of the set X ∩ Y is the set of elements that are members of the universal set U but not members of X ∩ Y. It is denoted by (X ∩ Y) ’.

The symmetric difference of two sets is the collection of elements which are members of either set but not both - in other words, the union of the sets excluding their intersection. Forming the symmetric difference of two sets is simple, but forming the symmetric difference of three sets is a bit trickier.

Example:

Suppose U = set of positive integers less than 10,

X = {1, 2, 5, 6, 7} and Y = {1, 3, 4, 5, 6, 8} .

a) Draw a Venn diagram to illustrate ( X ∩ Y ) ’

b) Find ( X ∩ Y ) ’

Solution:

a) First, fill in the elements for X ∩ Y = {1, 5, 6}

Fill in the other elements for X and Y and for U

Shade the region outside X ∩ Y to indicate (X ∩ Y ) ’

b) We can see from the Venn diagram that

(X ∩ Y ) ’ = {2, 3, 4, 7, 8, 9}

Or we find that X ∩ Y = {1, 5, 6} and so

(X ∩ Y ) ’ = {2, 3, 4, 7, 8, 9}