4.
[tex]\it \mathcal{A}_{disc}=\pi\cdot R^2 \Rightarrow R^2=\dfrac{\mathcal{A}}{\pi}\\ \\ \\ a)\ R^2=\dfrac{64\pi}{\pi}=64=8^2 \Rightarrow R=8\ m\\ \\ \\ b)\ R^2=\dfrac{81\pi}{\pi}=81=9^2 \Rightarrow R=9\ cm[/tex]
5.
[tex]\it L_{cerc}=d\cdot\pi \Rightarrow d=\dfrac{L}{\pi}\ \ (diametrul)\\ \\ \\ a)\ \ d=\dfrac{44\pi}{\pi}=44\ cm\\ \\ \\ b)\ \ d=\dfrac{12\pi}{\pi}=12\ m\\ \\ \\ c)\ \ d=\dfrac{77\pi}{\pi}=77\ dam[/tex]
6.
[tex]\it L_{cerc}=d\cdot\pi=2R\cdot\pi\\ \\ \\ \mathcal{A}_{disc}=\pi\cdot R^2 \Rightarrow R^2=\dfrac{\mathcal{A}}\pi\\ \\ \\ a)\ \ R^2=\dfrac{64\pi}{\pi}=64=8^2 \Rightarrow R=8cm\\ \\ L_{cerc}=2\cdot8\pi=16\pi\ cm[/tex]
