1. The problem statement, all variables and given/known data Find the input resistances and voltage gains for those ideal op amps 2. Relevant equations 3. The attempt at a solution The original inverting circuit look like this : we already have the equations : input resistance = 10k voltage gain = -r2/r1 = -10 For the first circuit : it still a inverting op amps, does the red marked 10k resistor get involved with input resistances ? I think it's not because it connected to the ground (virtual ?). R2 is 100k so the equation for voltage gain remains the same as the original circuit . Second circuit: there is no current in red marked 10k resistor, input resistance is unchanged (10k), voltage gain remains (-10k) I find it is difficult to calculate using op amps characteristics, can I use voltage node method to find the voltage gain, which node should I choose. Are those ground connected resistor have no effect on the circuit input resistances and voltage gain at all ?