Answer
$\text{The area of of a triangle varies jointly as the length of its base and its height.}$
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.
(3) When $y$ varies jointly as $x$ and $z$, joint variation's equation is $y=kxz$ where $k$ is the constant of variation.
In the equation $A=\frac{1}{2}bh$, $A$ varies jointly as $b$ and $h$ with a constant of variation of $\frac{1}{2}$.
Thus:
$\text{The area of of a triangle varies jointly as the length of its base and its height.}$