so I also need to state: x\notin B and x\notin C ?
So if I tagged those statements onto the proof after I state x\in A, then its good?
Im just a little confused, by saying:
\forall x\in (B\cup C), x\in B\, or\, x\in C \, or \, both
\therefore \, \, \forall x\, \in\begin{Bmatrix}...