[tex]\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 } \\\\ \textit{\underline{x} varies directly with }\underline{z^5}\qquad \qquad \stackrel{\textit{constant of variation}}{x=\stackrel{\downarrow }{k}z^5~\hfill } \\\\[-0.35em] ~\dotfill[/tex]
[tex]\stackrel{\textit{f(x) varies directly with "x"}}{f(x) = kx}\qquad \textit{we also know that} \begin{cases} f(x)=130\\ x = 5 \end{cases} \\\\\\ 130=k5\implies \cfrac{130}{5}=k\implies 26=k~\hfill \boxed{f(x)=26x} \\\\\\ \textit{when x = 11, what is f(x)?}~~f(x)=26(11)\implies f(x)=286[/tex]
