Prove that for all sets X and Y, (X ∩ Y) ∪(Y-X)=Y
I'm stuck and I'm not fully sure how to solve this.
suppose that a is an element of (X∩Y) U (Y-X).
this means one of two things:
a is in X∩Y, or
a is in Y-X
suppose a is in X∩Y. this means that a is in BOTH X and Y, so is certainly in Y.
alternatively, if a is in Y-X, this means a is in Y, but not in X. who cares if its not in X, at least it's in Y!
so in all possible cases, we see that a is in Y.
this means that (X∩Y) U (Y-X) is a subset of Y.
now suppose b is in Y.
well we have two possibilities:
b is in X
b is NOT in X (in or out, that's the way it is with sets. you cannot be "sort of" in a set).
if b is in X, then b is in X AND Y, so b is in X∩Y.
if b is not in X, then b is in Y, but not in X, so b is in Y-X.
if we put these two sets together, b is certain to be in one of them. hence b is in (X∩Y) U (Y-X).
thus Y is a subset of (X∩Y) U (Y-X).
but, if for 2 sets A,B: if A⊆B and B⊆A, then A and B have exactly the same elements, that is: A = B.
so (X∩Y) U (Y-X) = Y.
(intuitively what we are doing is splitting Y into 2 parts: the part that overlaps with X, and the part that doesn't).