[tex]\bf \qquad \qquad \textit{direct proportional variation} \\\\ \textit{\underline{y} varies directly with \underline{x}}\qquad \qquad \stackrel{\textit{constant of variation}}{y=\stackrel{\downarrow }{k}x~\hfill } \\\\[-0.35em] \rule{34em}{0.25pt}\\\\ (\stackrel{x}{5}~,~\stackrel{y}{8})\qquad \implies 8=k(5)\implies \cfrac{8}{5}=k[/tex]
