Answer
$s=kx^3$, means that $s$ varies directly with the cube of $x$.
Work Step by Step
The general equation of an inverse variation is $y=k(x)$ where $k$ is the constant of variation.
$s=k(x^3)$, means that $s$ varies directly with the cube of $x$, with $k$ as a constant of variation.
We can say that the strength of a muscle varies directly with the cube of its length.
