Write an equation that expresses the following relationship.
$p$ varies directly with $d$ and inversely with the square of $u$
In your equation, use $k$ as the constant of proportionality.

Step 1 ：The question is asking for an equation that expresses a relationship where $p$ varies directly with $d$ and inversely with the square of $u$. This means that as $d$ increases, $p$ also increases and as $u$ increases, $p$ decreases. The constant of proportionality, $k$, is a constant value that relates $p$, $d$, and $u$ in this relationship.

Step 2 ：In mathematical terms, a direct variation is expressed as $y = kx$, where $y$ varies directly as $x$. An inverse variation is expressed as $y = k/x$, where $y$ varies inversely as $x$.

Step 3 ：Given these, we can express the relationship as $p = kd/u^2$.

Step 4 ：The equation that expresses the relationship is \(\boxed{p = \frac{kd}{u^2}}\).