Show that the system of rational expressions is closed under addition, or give an example showing the contrary.

Let p/q, r/s be elements of Q , the set of rational numbers, where q and s are non zero.We need to show that the sum is also a rational number.p/q + r/s =  (p*s + q*r ) /(q*s)
Since q and s are non zero, the sum is a rational number.