$\left\vert{(A \cup B)}\right\vert = \left\vert{A}\right\vert + \left\vert{B}\right\vert - \left\vert{A \cap B}\right\vert = 5 + 4 - 2 = 7$
Definisjon: Inklusjon-og-eksklusjonsprinsippet for to mengder
Test deg selv
La $A = \{1,3,5,7,9\}$ og $B = \{1,2,3,4\}$. $\left\vert{A \cup B}\right\vert =$
Test deg selv
La $\left\vert{C}\right\vert = 4$, $\left\vert{D}\right\vert = 6$ og $\left\vert{(C \cup D)}\right\vert = 8$. $\left\vert{C \cap D}\right\vert =$
Fra inklusjon-og-eksklusjonsprinsippet har vi at $\left\vert{C \cup D}\right\vert = \left\vert{C}\right\vert + \left\vert{D}\right\vert - \left\vert{C \cap D}\right\vert$. Ved å løse ligningen for $\left\vert{C \cap D}\right\vert$ får vi at $\left\vert{C \cap D}\right\vert = \left\vert{C}\right\vert + \left\vert{D}\right\vert - \left\vert{C \cup D}\right\vert$.
$\left\vert{C \cap D}\right\vert = \left\vert{C}\right\vert + \left\vert{D}\right\vert - \left\vert{C \cup D}\right\vert = 4 + 6 - 8 = 2$