r_{1} = \sqrt{1 \times 4, 05 \cdot 10^{- 7} \times \frac{0, 5 \times 0, 5}{0, 5 + 0, 5}} = 3, 2 \cdot 10^{- 4} m = 320 μ m

${r_1=\sqrt{1 \times 4,05 \cdot 10^{-7} \times \frac{0,5×0,5}{0,5+0,5}}=3,2 \cdot 10^{-4}\, \mathrm{m}=320\, \mathrm{ \mu m}}$

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