= Solution
Apply the two identities already proved with $A=\mathcal U$. Every $B_i$ is a <subset> of the common universe, and a <complement> is the <set difference> from that universe. Therefore
$$
\mathcal U\setminus\bigcup_{i=1}^nB_i
=\bigcap_{i=1}^n(\mathcal U\setminus B_i),\qquad
\mathcal U\setminus\bigcap_{i=1}^nB_i
=\bigcup_{i=1}^n(\mathcal U\setminus B_i).
$$
Replacing each <set difference> from $\mathcal U$ by its <complement> notation gives the claimed finite forms of <De Morgan's laws>.
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