The inverting feedback divider gives $V_{out}=(1+R_F/R_1)V_{in}$ because the ideal inputs have equal voltage. Input current flows only through R to the output: $I_{in}=(V_{in}-V_{out})/R=-V_{in}R_F/(RR_1)$. Therefore Zin=Vin/Iin=−RR₁/RF, B, a negative input resistance.
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