Concept:
De Morgan's theorem is used to simplify complemented Boolean expressions.
Step 1: The theorem states:
\[
\overline{A+B}=\overline{A}\cdot\overline{B}
\]
Step 2: It means the complement of OR operation becomes AND operation of complements.
Step 3: Therefore:
\[
\overline{A+B}=\overline{A}\,\overline{B}
\]
Hence:
\[
\boxed{\overline{A}\,\overline{B}}
\]