a/b + c/d = (a*d + b*c)/(b*d)
Closure property states:
If x and y are any two integers, xy will also be an integer:
Therefore:
a * b = integer
b * c = integer
b * d = integer
Also, addition and subtraction states that the sum or difference of any two integers will always be an integer too
So:
a*d + b*c = integer
We can conclude according to the previous information that:
a/b + c/d = (a*d + b*c)/(b*d) = rational number
Or, in another words:
the sum of two rational numbers is rational