Answer
$A=\displaystyle \frac{1}{2}ab$
Work Step by Step
$y$ varies directly with $x,$ or $y$ is directly proportional to $x$,
if there is a nonzero number $k$ such that$ y=kx.$
The number $k$ is called the constant of proportionality.
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Here, y is "the area, A", and
x = "product of b and h"
The given model is
$ A=k(ab)\quad$
Given that $k=\displaystyle \frac{1}{2},$ it means that
$A=\displaystyle \frac{1}{2}ab$