Respuesta :

[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}\\\\ -------------------------------\\\\ \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]