To add or subtract expressions, expand them to make their denominators the same. Least common multiple of $p^{2}$ and $q^{2}$ is $p^{2}q^{2}$. Multiply $\frac{1}{p^{2}}$ times $\frac{q^{2}}{q^{2}}$. Multiply $\frac{1}{q^{2}}$ times $\frac{p^{2}}{p^{2}}$.
