[tex]\it \mathcal{A}= \pi R^2 \Rightarrow R^2=\dfrac{\mathcal{A}}{\pi} \Rightarrow R=\sqrt{\dfrac{\mathcal{A}}{\pi}}\\ \\ \\ L_c=2\pi R=2\pi\cdot\sqrt{\dfrac{\mathcal{A}}{\pi}}[/tex]
[tex]\it a)\ \ L_c=2\pi\cdot\sqrt{\dfrac{\mathcal{A}}{\pi}}=2\pi\cdot\sqrt{\dfrac{64\pi}{\pi}}=2\pi\sqrt{64}=2\pi\cdot8=16\pi\ cm[/tex]
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