What is Disjoint sets ?

The pair of disjoint sets are

If A and B are two disjoint sets, then which one of the following is correct?

If A and B are two disjoint sets n(A)+n(B)=24 then find n(A cup B)

There are two disjoint sets S_1 and S_2 where S_1={f(1),f(2),f(3)…..........} S_2={g(1),g(2),g(3)….........} such that S_1 uu S_2 forms the set of natural numbers. Also f(1) lt f(2) lt f(3)…........ & g(a) lt g(2) lt g(3) and f(n)=g(g(n))+1 then what is g (1) ?

If A and B are disjoint sets, then n (^^DeltaB) = "____" .

If A and B are disjoint sets, then n(A cap B) = ?

If a and B are disjoint sets, then n(A uu B)= _______

If A and B are disjoint sets, then write A nn B