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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.

3 Answers 3