$ \text{Gegeben:}\\\text{Oberfläche} \qquad O \qquad [m^{2}] \\ \text{Kreiszahl} \qquad \pi \qquad [] \\ \\ \text{Gesucht:} \\ \text{Radius} \qquad r \qquad [m] \\ \\ r = \sqrt{\frac{ O}{\pi \cdot 4}}\\ \textbf{Gegeben:} \\ O=1\frac{2}{5}m^{2} \qquad \pi=3\frac{16}{113} \qquad \\ \\ \textbf{Rechnung:} \\ r = \sqrt{\frac{ O}{\pi \cdot 4}} \\ O=1\frac{2}{5}m^{2}\\ \pi=3\frac{16}{113}\\ r = \sqrt{\frac{ 1\frac{2}{5}m^{2}}{3\frac{16}{113} \cdot 4}}\\\\r=0,334m \\\\\\ \small \begin{array}{|l|} \hline O=\\ \hline 1\frac{2}{5} m^2 \\ \hline 140 dm^2 \\ \hline 1,4\cdot 10^{4} cm^2 \\ \hline 1,4\cdot 10^{6} mm^2 \\ \hline 0,014 a \\ \hline 0,00014 ha \\ \hline \end{array} \small \begin{array}{|l|} \hline r=\\ \hline 0,334 m \\ \hline 3,34 dm \\ \hline 33,4 cm \\ \hline 334 mm \\ \hline 3,34\cdot 10^{5} \mu m \\ \hline \end{array}$