[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}\\\\
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\textit{we also know that }(~\stackrel{x}{-8}~,~\stackrel{y}{10}~)\implies 10=k(-8)
\\\\\\
\cfrac{10}{-8}=k\implies \cfrac{5}{-4}=k\qquad therefore\qquad \boxed{y=-\cfrac{5}{4}x}[/tex]
