I need to prove following formula:
$|A \cup B|=|A|+|B|-|A\cap B|$
I got so far:
$A=(A\cap B) \cup (A \cap !B)$ // !B should be the complement of B
With this I can get to
$|A|=|A\cap B| + |A \cap !B|$
But I do not have any clue on how to go on. Some advises would be nice.