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.