# Set proof

#### putiiik

##### New member
Prove that (A⊕B)∩A= A-B! Thank you!

#### Evgeny.Makarov

##### Well-known member
MHB Math Scholar
There are various ways of proving this: using the fundamental identities of set algebra, using Euler-Venn diagrams, by definition using mutual inclusion of the left- and right-hand sides, etc. Which one is used in your course? And if it is the first method, are you familiar with the fundamental identities?

#### Country Boy

##### Well-known member
MHB Math Helper
For arbitrary sets, union $A\cup B$, intersection $A\cap B$, and difference A\ B, are defined but how are you defining the "direct sum" $A\bigoplus B$ of sets?

MHB Math Scholar

#### Country Boy

##### Well-known member
MHB Math Helper
It seems very strange to us a "+" symbol to mean a "difference".

#### Klaas van Aarsen

##### MHB Seeker
Staff member
It seems very strange to us a "+" symbol to mean a "difference".
It's the union of both sets except for their intersection.
As such a "+" seems appropriate.
It's just that to define it, we typically take the union of the 2 mutual differences, which is apparently why it is called symmetric difference.