Bonferroni's Inequality states that:

But the first axiom of probability states that:

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Am I allowed to do the following?

And the last line must be true because the entire sample size must be at least equal to the union of A and B minus their intersection.

OR

Should i rather try to prove it this way:

Rearrange so that the 1 is isolated...

Which is also true...

Anyone got an idea which one of these is correct?

Thanks in advance.

EDIT: The second one might be the correct one seeing as A and B are not disjoint. Am i right?