
Proving Sets
A and B are sets i have to show proof that the following are equivalent
(i) A(is a subset of ) B
(ii) A(intersect)B = A
(iii) A(union) B = B
*note with (i) the symbol used is the subset symbol with a line underneath and im not sure if that means A=B or not.
Thanks

Note that as (i) implies that A is a subset of B, then it follows that the intersection of B with A equals A, and it also follows that the union of A with B is B. The other way round : if the union of A with B is B, then A must be a subset of B, and it follows that the intersection of B with A equals A.
To help understand this, draw a diagram with and :)
Once you will have understood why this works, you will be able to put it in mathematical terms.
Here are some basic ideas that can help you, too :
If A is a subset of B, it means that all the elements of A are contained in B, among other elements of B
If A intersect B = A, it means that the only elements that are in A and in B are the elements of B (equivalent to : A is a subset of B)
If A union B = B, it means that the elements that are in A or in B, are equal to the elements of B (equivalent to : A is a subset of B)

thank you.
I understand the relationships between the different sets, however I am unsure as to how it should be represented as a proof.
Using what you said I can write:
(i) implies that A is a subset of B and if x (is in) A then x (is in) B
A(intersection) B = A implies that
if x (is in) A and x (is in) B then x (is in) B = A(is a subset of) B
A(union)B = B implies that
if x (is in) A or x (is in) B then x (is in) B = A(is a subset of) B
therefore (i), (ii), (iii) are equivalent
is this a proof or not?

Sketched proof :
(1) If A is a subset of B, then A intersect B = A
(2) If A is a subset of B, then A union B = B
(3) Therefore, by transitivity, if A is a subset of B then A intersect B = A and A union B = B.
You only need to prove the weaker statements (1) and (2). Here is an example for (1) :
Statement : If A is a subset of B, then A intersect B = A
Proof : A is a subset of B, therefore each element of A is in B. This also implies that there exists no element of A which is not in B, and it follows that if A is a subset of B, then A intersect B = A.
(2) can be proven in a similar way, and once you will have proven it you will have proven your original problem.

how is this for proving (2):
A is a subset of B
therefore if x is in A then x is in B
this also implies that if x is in A or x is in B then x is in B.
and it follows that if A is a subset of B then A union B = B
the proof sounds a bit off to me

Yes, it is correct. Check out this one.
Statement : If A is a subset of B, then A union B = B
Proof : A union B returns the set of elements that are in A or in B. Say that A is a subset of B. It follows that any element that is in A is in B, and that the elements of B that are not in A are only in B. It immediately follows that A union B = B.
Of course you can do it using letters, it equally works.
