A2: Q = Cl Q by PRE_TOPC:22;
P = Cl P by PRE_TOPC:22;
then Cl (P \/ Q) = P \/ Q by A2, PRE_TOPC:20;
hence for b₁ being Subset of TS st b₁ = P \/ Q holds
b₁ is closed ; :: thesis: verum