Final answer:
The average of two positive integers is positive, the product of two negative integers is positive, and the difference of two negative integers is not necessarily negative.
Step-by-step explanation:
a. The average of two positive integers is positive can be expressed as:
∀x, y ∈ Z+ (x + y)/2 > 0
b. The product of two negative integers is positive can be expressed as:
∀x, y ∈ Z- xy > 0
c. The difference of two negative integers is not necessarily negative can be expressed as:
∃x, y ∈ Z- x - y ≥ 0