[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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\stackrel{\textit{\underline{A} varies directly proportional with the square of \underline{r}}}{A=kr^2\qquad \boxed{k=1}\qquad A=1r^2\implies A=r^2}[/tex]
