6. Use quantified predicates to express the following sentences:
a. ∀x, y ∈ Z, x < 0 ∧ y < 0 → x + y < 0
b. ∀x ∈ Z⁺, ¬∃y ∈ Z, y = -x
c. ∀x, y ∈ Z, ∃z ∈ Z, z = x * y
7. Negate each predicate you created in question 6, then simplify them to remove the negation operators:
a. ∃x, y ∈ Z, x < 0 ∧ y < 0 ∧ x + y ≥ 0
b. ∃x ∈ Z⁺, ∀y ∈ Z, y ≠ -x
c. ∃x, y ∈ Z, ∀z ∈ Z, z ≠ x * y