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Step-by-step explanations:
Since the domain is a set of all real numbers, all the expression are with respect to real numbers.
a. 3 x,y : x/y < 1 ∧ y/x < 1
b. ∀x > 0, 1/x > 0
c. 3 x,y : x + y = xy
d. ∀x > 0 ∧ y > 0, x/y > 0 ∧ y/x > 0.
3 means "there exists"
∀ means "for every value of"
∧ means "and"