Exercise 3 Page 526

When one quantity varies with respect to two or more quantities, we have a combined variation. When one quantity varies directly with two or more quantities, we have what is called a joint variation.

Combined Variation	Equation Form
$z$ varies jointly with $x$ and $y .$	$z = k x y$
$z$ varies jointly with $x$ and $y,$ and inversely with $w .$	$z = \frac{k x y}{w}$
$z$ varies directly with $x$ and inversely with the product $w y .$	$z = \frac{k x}{w y}$

In our exercise, we are told that

z

varies inversely with the product of

x

and

y .