Find the value of $dy/dx$ at $x=8$ 
Given that variables $xy=40$, find $dy/dx$ at $x=8$.

I used $40/8$ to get $y=5$.
So why is the answer $-5/8$ and not $5/8$?

$xy=40\Rightarrow8y=40\Rightarrow y=5$
$xy=40\Rrightarrow xdy+ydx=0\Rightarrow \frac{dy}{dx}=-\frac{y}{x}=-\frac{5}{8}$
