[tex]\bf \qquad \qquad \textit{direct proportional variation}\\\\
\textit{\underline{y} varies directly with \underline{x}}\qquad \qquad y=kx\impliedby
\begin{array}{llll}
k=constant\ of\\
\qquad variation
\end{array}\\\\
-------------------------------[/tex]
[tex]\bf \textit{we also know that }
\begin{cases}
y=5.4\\
x=9
\end{cases}\implies 5.4=k9\implies \cfrac{5.4}{9}=k
\\\\\\
\cfrac{\quad \frac{54}{10}\quad }{9}=k\implies \cfrac{54}{10}\cdot \cfrac{1}{9}=k\implies \cfrac{3}{5}=k\qquad thus\qquad \boxed{y=\cfrac{3}{5}x}
\\\\\\
\textit{when x = -10, what is \underline{y}?}\qquad y=\cfrac{3}{5}(-10)[/tex]
