[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}\\\\
-------------------------------\\\\
\begin{cases}
y=9\\
x=3
\end{cases}\implies 9=k3\implies \cfrac{9}{3}=k\implies \boxed{3=k}
\\\\\\
thus\qquad y=3x[/tex]
