How to show in propositional logic, that $\mathrm{Cn}(\mathrm{Cn}(A)) = \mathrm{Cn}(A)$?
I thought of first showing $\mathrm{Cn}(\mathrm{Cn}(A)) \subseteq \mathrm{Cn}(A)$ and then $\mathrm{Cn}(\mathrm{Cn}(A)) \supseteq \mathrm{Cn}(A)$. I managed to show the second but don't exactly know how to show the first inclusion.