Answer
$\text{The surface area of a sphere varies directly as the square of its radius.}$
Work Step by Step
Recall:
(1) When $y$ varies directly as $x$, the direct variation's equation is $y=kx$ where $k$ is the constant of variation.
(2) When $y$ varies inversely as $x$, the inverse variation's equation is $y=\frac{k}{x}$ or
$xy=k$ where $k$ is the constant of variation.
In the equation $S=4\pi{r^2}$, $S$ varies directly as $r^2$ with a constant of variation of $4\pi$.
Thus:
$\text{The surface area of a sphere varies directly as the square of its radius.}$
