College Algebra 7th Edition

$\displaystyle R= \frac{kP^{2}t^{2}}{b^{3}}$
Since $R$ is directly (jointly) proportional to the squares of $P$ and $t$, but inversely proportional to the cube of $b$, we have: $\displaystyle R= \frac{kP^{2}t^{2}}{b^{3}}$ (where $k$ is a constant.)