Hey guys, I need some help on this problem please

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Verify the following extension of the addition rule by a) an appropriate Venn diagram and b) by a formal argument using the axioms of probability and the propositions in the chapter.

$\displaystyle P(A \cup B \cup C) = P(A) + P(B) + P(C) - P(A \cap B) - P(A \cap C) - P(B \cap C) + P(A \cap B \cap C)$

I don't understand the last part which says: $\displaystyle + P(A \cap B \cap C)$ because they are now adding some of the same elements twice. But the question does say VERIFY and not prove...

Thanks in advance.