**What is the inclusion-exclusion principle for 4 sets?**
https://math.stackexchange.com/questions/688019/what-is-the-inclusion-exclusion-principle-for-4-sets

WEBFeb 19, 2015 · 6. You could intuitively try to prove an equation by drawing four sets in the form of a Venn diagram -- say A1,A2,A3,A4 A 1, A 2, A 3, A 4, and observing the intersections between the circles. You want to find the cardinality of the union. Now, you will notice that if you just try to add the four sets, there will be repeated elements.

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