In general:
1) The negation of ∀x(P(x)) is ∃x(¬P(x)).
2) The negation of ∃x(P(x)) is ∀x(¬P(x)).
So we have:
a) The negation of ∀x(x>1) is ∃x(¬(x>1))=∃x(x≤1).
b) The negation of ∀x(x≤2) is ∃x(¬(x≤2))=∃x(x>2).
c) The negation of ∃x(x≥4) is ∀x(¬(x≥4))=∀x(x<4).
d) The negation of ∃x(x<0) is ∀x(¬(x<0))=∀x(x≥0).
e) The negation of ∀x((x<−1)∨(x>2)) is ∃x(¬((x<−1)∨(x>2)))=∃x((x≥−1)∧(x≤2))=
∃x(−1≤x≤2).
f) The negation of ∃x((x<4)∨(x>7)) is
∀x(¬((x<4)∨(x>7)))=∀x((x≥4)∧(x≤7))=
=∀x(4≤x≤7).
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