R = \frac{(R_{A C} + R_{3}) \cdot R_{4}}{R_{A C} + R_{3} + R_{4}} \approx 1 Ω

${R = \frac{(R_{AC}+R_3)\cdot R_4}{R_{AC}+R_3+R_4} \approx {1}{\Omega}}$

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