[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}\\\\
-------------------------------\\\\
(\stackrel{x}{8}~~,~~\stackrel{y}{4})\implies
\begin{cases}
x=8\\
y=4
\end{cases}\implies 4=k8\implies \cfrac{4}{8}=k\implies \cfrac{1}{2}=k
\\\\\\
\boxed{y=\cfrac{1}{2}x}[/tex]
