ρ₁ = 0,028 Ом·мм²/м
ρ₂ = 0,1 Ом·мм²/м
L₁ = 1000 м
S₁ = 4 мм²
S₂ = 2 мм²
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L₂ - ?
\ \ \ \ \frac{\rho_{1}\cdot L_{1}}{S_{1}}=\frac{\rho_{2}\cdot L_{2}}{S_{2}}\\\\\\{} \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ L_{2}=\frac{\rho_{1}\cdot L_{1}\cdot S_{2}}{\rho_{2}\cdot S_{1}}=\frac{0,028\cdot1000\cdot2}{0,1\cdot4}=140 \ (m)" alt="\displaystyle \tt R_{1}=R_{2} \ \ \ \ => \ \ \ \ \frac{\rho_{1}\cdot L_{1}}{S_{1}}=\frac{\rho_{2}\cdot L_{2}}{S_{2}}\\\\\\{} \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ L_{2}=\frac{\rho_{1}\cdot L_{1}\cdot S_{2}}{\rho_{2}\cdot S_{1}}=\frac{0,028\cdot1000\cdot2}{0,1\cdot4}=140 \ (m)" align="absmiddle" class="latex-formula">
Ответ: 140 м.