# Prove property of symmetric difference: if $A \triangle B \subseteq A$ then $B \subseteq A$

This problem is from Velleman p143 5.

Recall from Section 1.4 that the symmetric difference of two sets A and B is the set $A \triangle B = (A \setminus B) \cup (B \setminus A) = ( A \cup B) \setminus (A \cap B)$. Prove that if $A \triangle B \subseteq A$ then $B \subseteq A$.

My strategy: The crux of this proof is to reduce $( A \cup B) \setminus (A \cap B)$ down to the fundamental logical form. $(A \setminus B) \cup (B \setminus A) = (x \in A \land x \notin B) \lor (x \in B \land x \notin A)$. This gives us a disjunction where if either case is true, then $x \in A$.

My proof:

Let x be an arbitrary member of B. Suppose that $A \triangle B \subseteq A$. All the members of $A \triangle B$ are in either A or B, but not both. The case where $x \in A \land x \notin B$ is not applicable since $x \in B$. That leaves the case where $(x \in B \land x \notin A)$. Since $x \in B$, we know that $x \in A \triangle B$ and we're given this means that $x \in A$. Therefore $x \in B \to x \in A$. Since x was arbitrary, we assert that $B \subseteq A$.

My issue: The proof seems OK, I think. The Velleman Proof Designer seems to jive with my argument. However, it's really bothering me that the logical decomposition involves $(x \in B \land x \notin A ) \to x \in A$. How can x be a member of A and at the same time not be a member of A?

• If $A \triangle B \subset A$, then you must have $B \setminus A = \emptyset$. Hence $B \subset A$. – copper.hat Jan 6 '15 at 23:43
• That helps. A = {1, 2, 3, 4}. B = {4}. $B \setminus A = \emptyset$. $A \triangle B = \{1, 2, 3 \}\subseteq A$ is true. $B \subseteq A$ is also true. This all works. I guess I just need to accept $(x \in B \land x \notin A) \to x \in A$ as a strange consequence of the logic. – Marty B. Jan 7 '15 at 16:00
• Think of it this way, $A \triangle B$ is made of two parts, (i) parts of $A$ that are not in $B$ and (ii) parts of $B$ that are not in $A$. If $A \triangle B \subset A$, then the (ii) parts must be empty or you have an immediate contradiction, and so $B \setminus A$ is empty. – copper.hat Jan 7 '15 at 19:04
• – Arnaud D. Oct 26 '18 at 12:46

You are right that it is not possible for "$x$ [to be] be a member of $A$ and at the same time not be a member of $A$". That is in fact the point.
Assume $x\in B$. Then by the statement $(x \in B \land x \notin A ) \to x \in A$, if $x$ is not in $A$ then it is in $A$. That is a contradiction, so our hypothesis "$x$ is not in $A$" must be false. Therefore, $x\in A$.
That last paragraph shows $x\in B \implies x\in A$. Therefore, $B\subseteq A$. That is the very last statement of the argument.