R＝ρL/S,电阻决定式.R＝5Ω
长度L＝50m,横截面积S＝4mm²＝4×10^-6m²
所以ρ＝RL/S＝5Ω×50m/(4×10^-6m²)＝6.25×10^7Ω·m