\frac{ρ_{1}}{ρ_{2}} = \frac{Δ U_{1} l_{2}}{Δ U_{2} l_{1}}, \frac{ρ_{1}}{ρ_{3}} = \frac{Δ U_{1} l_{3}}{Δ U_{3} l_{1}}, \frac{ρ_{2}}{ρ_{3}} = \frac{Δ U_{2} l_{3}}{Δ U_{3} l_{2}}

${\frac{\rho_1}{\rho_2}=\frac{\Delta U_1l_2}{\Delta U_2l_1}, \quad \quad \frac{\rho_1}{\rho_3}=\frac{\Delta U_1l_3}{\Delta U_3l_1}, \quad \quad \frac{\rho_2}{\rho_3}=\frac{\Delta U_2l_3}{\Delta U_3l_2}}$

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