\(\displaystyle\left(\bigcap\limits_{i=1}^nA_i\right)^C\)
\(\displaystyle=\{a:a\in\left(\bigcap\limits_{i=1}^nA_i\right)^C\wedge a\notin\bigcap\limits_{i=1}^nA_i\}\)
\(\displaystyle=\{a:a\in\left(\bigcap\limits_{i=1}^nA_i\right)^C\wedge\lnot\left(\forall i\in\{1,...,n\}:a\in A_i\right)\}\)
\(\displaystyle=\{a:a\in\left(\bigcap\limits_{i=1}^nA_i\right)^C\wedge\exists i\in\{1,...,n\}:a\notin A_i\}\)
\(\displaystyle=\{a:\exists i\in\{1,...,n\}:a\in\left(\bigcap\limits_{i=1}^nA_i\right)^C\wedge a\notin A_i\}\)
\(\displaystyle=\bigcup\limits_{i=1}^nA_i^C\)