Answer:
First, a rational number is defined as the quotient between two integer numbers, such that:
N = a/b
where a and b are integers.
Now, the axiom that we need to use is:
"The integers are closed under the multiplication".
this says that if we have two integers, x and y, their product is also an integer:
if x, y ∈ Z ⇒ x*y ∈ Z
So, if now we have two rational numbers:
a/b and c/d
where a, b, c, and d ∈ Z
then the product of those two can be written as:
(a/b)*(c/d) = (a*c)/(b*d)
And by the previous axiom, we know that a*c is an integer and b*d is also an integer, then:
(a*c)/(b*d)
is the quotient between two integers, then this is a rational number.
