\[\left( 5m^{3}n^{2} + 8m^{2}n^{2} \right)\ :mn =\]
\[= \frac{5m^{3}n^{2}}{\text{mn}} + \frac{8m^{2}n^{2}}{\text{mn}} =\]
\[= 5m^{2}n + 8mn.\]