Answer :
[tex]\bf \qquad \qquad \textit{direct proportional variation}
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\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 }
\begin{cases}
y=18\\
x=\frac{1}{2}
\end{cases}\implies 18=k\cfrac{1}{2}\implies 18=\cfrac{k}{2}
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36=k\qquad therefore\qquad \boxed{y=36x}[/tex]