Given n sets a₁, a₂. Aₙ. The inclusion-exclusion principle states that the cardinatlity of a₁∪ a₂∪. ∪aₙ is the sum of the individual cardinalities minus the sum of the cardinalities of intersections of any two sets plus the sum of the cardinalities of intersections of any three sets minus the sum of the cardinalities of intersections of any four sets and so on. This alternating sum ends with the cardinality of the intersection of all n sets. How many terms are there in this formula that are intersections of 3 sets?.