# How do you simplify 4/(5n)-1/(10n^3)?

We multiply the left term by $1$, disguised as $\frac{2 {n}^{2}}{2 {n}^{2}}$
$= \frac{4 \times 2 {n}^{2}}{5 n \times 2 {n}^{2}} - \frac{1}{10 {n}^{3}} = \frac{8 {n}^{2}}{10 {n}^{3}} - \frac{1}{10 {n}^{3}} = \frac{8 {n}^{2} - 1}{10 {n}^{3}}$