# Set Difference as Symmetric Difference with Intersection

## Theorem

$S \setminus T = S * \paren {S \cap T}$

where:

$S \setminus T$ denotes set difference
$S * T$ denotes set symmetric difference
$S \cap T$ denotes set intersection.

## Proof

 $\displaystyle S * \paren {S \cap T}$ $=$ $\displaystyle \paren {S \setminus \paren {S \cap T} } \cup \paren {\paren {S \cap T} \setminus S}$ Definition of Symmetric Difference $\displaystyle$ $=$ $\displaystyle \paren {S \setminus \paren {S \cap T} } \cup \O$ Set Difference of Intersection with Set is Empty Set $\displaystyle$ $=$ $\displaystyle \paren {S \setminus T} \cup \O$ Set Difference with Intersection is Difference $\displaystyle$ $=$ $\displaystyle S \setminus T$ Union with Empty Set

$\blacksquare$