R_{k} = ρ (\frac{l}{S} + \frac{Δ l}{S_{0}}) = ρ (\frac{l}{0, 7 S_{0}} + \frac{Δ l}{S_{0}}) \approx 17, 3 \frac{ρ}{S_{0}}

${R_k = \rho\left (\frac{l}{S}+\frac{\Delta l}{S_0}\right) = \rho\left(\frac{l}{0,7S_0}+\frac{\Delta l}{S_0}\right) \approx 17,3\frac{\rho}{S_0}}$

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