\[\boxed{\mathbf{6.71}}\]
\[\pi \approx 3,14\]
\[C = 2\pi R\]
\[R = \frac{C}{2\pi} = \frac{C}{6,28}\]
\[\textbf{а)}\ C = 12,56\ мм:\]
\[R = \frac{12,56}{6,28} = \frac{1256}{628} = 2\ мм.\]
\[\textbf{б)}\ C = 3,14\ см:\]
\[R = \frac{3,14}{2 \cdot 3,14} = \frac{1}{2} = 0,5\ см.\]
\[\textbf{в)}\ C = 0,0628\ м:\]
\[R = \frac{0,0628}{6,28} = \frac{6,28}{628} = 0,01\ м.\]