Given the Sets A and B, how do you prove that $|A \cup B| = |A| + |B| - |A \cap B|$
I know that if the sets are pairwise disjoint, the last term would be 0 and hence not be necessary for the equation. Hence I think you have to assume the sets are not pairwise disjoint. It seems like a very basic equation and I understand what it means but I am not sure how to prove it and the necessary theorems needed.
I would appreciate any help I can get.