(A∪B)'=A'∩B '。
Proof: let x∈(A∪B)', then x does not belong to A∪B,
∴x does not belong to a, x does not belong to b,
∴x∈A'∩B',
∴(A∪B)' is a subset of A'∩B'.
Similarly, it can be proved that A'∩B' is a subset of (A∪B)'.
∴(A∪B)'=A'∩B'.