[tex]\bf \textit{circumference of a circle}\\\\
C=2\pi r\qquad \qquad \implies \cfrac{C}{2\pi }=\boxed{r}\\\\
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\textit{area of a circle}\\\\
A=\pi r^2\qquad \qquad \implies A=\pi \left( \boxed{\cfrac{C}{2\pi }} \right)^2\implies A=\pi \left( \cfrac{C^2}{(2\pi )^2} \right)
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A=\cfrac{\pi C^2}{2^2\pi^2}\implies A=\cfrac{C^2}{4\pi }[/tex]