The negation of (existsx)(P(x)landnegQ(x))(\\exists x)(P(x) \\land \\neg Q(x))(existsx)(P(x)landnegQ(x)) is equivalent to:
(forallx)(P(x)rightarrowQ(x))(\\forall x)(P(x) \\rightarrow Q(x))(forallx)(P(x)rightarrowQ(x))
Both a and b
Neither a nor b
(forallx)(negP(x)lorQ(x))(\\forall x)(\\neg P(x) \\lor Q(x))(forallx)(negP(x)lorQ(x))