Editing Inclusion–exclusion principle (section)

===Counting intersections===
The principle of inclusion–exclusion, combined with [[De Morgan's law]], can be used to count the cardinality of the intersection of sets as well. Let <math>\overline{A_k}</math> represent the complement of ''A<sub>k</sub>'' with respect to some universal set ''A'' such that <math>A_k \subseteq A</math> for each ''k''. Then we have

:<math>\bigcap_{i=1}^n A_i = \overline{\bigcup_{i=1}^n \overline{A_i}}</math>

thereby turning the problem of finding an intersection into the problem of finding a union.