Basic properties of symmetric difference in sets: $A \Delta B=A \Delta D$ implies $B=D$ How can I prove that for any sets $A,B,C,D$
if 
$$A \Delta B = C  $$
and
$$A \Delta D = C  $$
then $B=D$.
Where $\Delta$ denotes symmetric difference.
 A: First show that $\Delta$ is associative and has inverses; namely, every set is its own inverse. Then apply $A$ from the left to both equations and use what you proved in the first step. In doing this, it may help to think of the symmetric difference as XOR.
A: The probabilist in me cannot resist mentioning that symmetric differences can (and maybe they should) be approached through indicator functions modulo 2. 
Recall that, for every $A\subseteq E$, the indicator function $\mathbf 1_A$ is defined on $E$ by $\mathbf 1_A(x)=1$ if $x$ is in $A$ and $\mathbf 1_A(x)=0$ if $x$ is in $E\setminus A$.
Then, for every subsets $A$ and $B$ of $E$, the set $C=A\Delta B$ is uniquely characterized by the fact that $\mathbf 1_C=\mathbf 1_A+\mathbf 1_B\pmod{2}$ (can you check this?).
A host of properties of $\Delta$ become obvious with this approach since $\Delta$ is simply the sum in the group $\mathbb Z/2\mathbb Z$. For example, your hypothesis is that $\mathbf 1_A+\mathbf 1_B=\mathbf 1_C=\mathbf 1_A+\mathbf 1_D\pmod{2}$ hence $\mathbf 1_B=\mathbf 1_D\pmod{2}$. And this is enough to guarantee that $B=D$ (can you check this?).
Note equivalently that, in every group $(G,\ast)$, the identity $g\ast h_1=g\ast h_2$ for some $g$, $h_1$ and $h_2$ in $G$ implies that $g^{-1}\ast g\ast h_1=g^{-1}\ast g\ast h_2$ hence $h_1=h_2$. Homework: what is $g^{-1}$ in the setting of the exercise?
A: $$B=\emptyset\Delta B=(A\Delta A)\Delta B=A\Delta(A\Delta B)=A\Delta C=A\Delta(A\Delta D)=(A\Delta A)\Delta D=\emptyset\Delta D=D$$
A: Assume $x$ is in $B$, and deduce from the given equations that it must also be in $D$. Then assume it's in $D$, and deduce by symmetry that it's in $B$. That will prove $B=D$. 
A: Take $b\in B$:
1.: $b\notin A$. This means that $b\in C\setminus A=D\setminus A$ and hence in $D$.
2.: $b\in A$. This means that $b\in A\setminus C=A\cap D$ and hence in $D$.
This proves that $B\subseteq D$ and for the other inclusion just swap $B$ and $D$.
A: $
\newcommand{\calc}{\begin{align} \quad &}
\newcommand{\calcop}[2]{\\ #1 \quad & \quad \text{"#2"} \\ \quad & }
\newcommand{\endcalc}{\end{align}}
$As a first simplification, we can take out $\;C\;$ and rephrase your theorem as
$$
\tag{0} A \Delta B = A \Delta D \;\Rightarrow\; B = D
$$
The proof depends on your definition and the laws of logic that are available.
In my favorite setting the proof would go something like this:
$$\calc
A \Delta B = A \Delta D
\calcop{\equiv}{set extensionality; definition (1) of $\;\Delta\;$, below, twice}
\langle \forall x :: x \in A \not\equiv x \in B \;\equiv\; x \in A \not\equiv x \in D \rangle
\calcop{\equiv}{logic: $\;\equiv\;$ is symmetric}
\langle \forall x :: x \in A \not\equiv x \in A \;\equiv\; x \in B \not\equiv x \in D \rangle
\calcop{\equiv}{logic: $\;\not\equiv\;$ is anti-reflexive}
\langle \forall x :: \text{false} \;\equiv\; x \in B \not\equiv x \in D \rangle
\calcop{\equiv}{logic: the negation of $\;\not\equiv\;$ is $\;\equiv\;$}
\langle \forall x :: x \in B \equiv x \in D \rangle
\calcop{\equiv}{set extensionality}
B = D
\endcalc$$
This completes the proof of (a slightly stronger version of) $(0)$.
Above we used the fact (or alternative definition) that for every $\;x\;$
$$
\tag{1} x \in X \Delta Y \;\equiv\; x \in X \not\equiv x \in Y
$$
Also, we implicitly used the fact that $\;\equiv\;$ and $\;\not\equiv\;$ are not only both associative but also mutually associative.
