d)
[tex]\it L_c=2\pi R=16\pi\bigg|_{:2\pi} \Rightarrow R=8\ cm\\ \\ \\ \mathcal{A}_{disc}=\pi R^2=\pi\cdot 8^2=64\pi\ cm^2[/tex]
c)
[tex]\it L_c=2\pi R=20\bigg|_{:2} \Rightarrow \pi R=10 \Rightarrow R=\dfrac{10}{\pi}\\ \\ \\ \mathcal{A}_{disc}=\pi R^2=\pi \cdor\dfrac{10}{\pi}\cdot\dfrac{10}{\pi}=\dfrac{100}{\pi}\ cm^2[/tex]
