# Proving Intersections and Unions

• September 1st 2012, 10:06 AM
lovesmath
Proving Intersections and Unions
(A intersect B) is contained in A is contained in (A union B)

I know I have to show containment both ways, meaning
(A intersect B) is contained in A
A is contained in (A union B)

Can you help me get started?
• September 1st 2012, 10:32 AM
emakarov
Re: Proving Intersections and Unions
To prove that X is contained in Y, show that every element of X is an element of Y. Use the definitions of union and intersection.
• September 1st 2012, 10:36 AM
Plato
Re: Proving Intersections and Unions
Quote:

Originally Posted by lovesmath
(A intersect B) is contained in A is contained in (A union B)

I know I have to show containment both ways, meaning
(A intersect B) is contained in A
A is contained in (A union B)
Can you help me get started?

If $x\in A\cap B$ then $x\in A~\&~x\in B$ therefore $x\in A$. That means $A\cap B\subseteq A.$
• September 1st 2012, 10:50 AM
HallsofIvy
Re: Proving Intersections and Unions
The way you prove " $X\subseteq Y$" is to start "if x is in X" then use the definitions of X and Y to show "then x is in Y". As Plato said, " $A\cap B$" is defined as the set of all x such that x is in A and x is in B. If $x\in A\cap B$ then $x\in A$. Now, you need to show "if $x\in A$ then $x\in A\cup B$". So, what is the definition of " $A \cup B$"?