In analysing an ideal op-amp circuit I'm asked to state the input resistance seen by an input voltage.
Some of this may be irrelevant but a quick summary of the circuit:
Two unknown voltages, VinA and VinB are connected to the inverting and non-inverting inputs, respectively. Both have a 10k resistor between Vin and the input. Vout feeds back to the inverting input, through another 10k resistor. VinB, after the first 10k resistor, is connected in parallel to the non-inverting input (as stated) and another 10k resistor which continues to ground.
So, the question is what is the input resistance seen by VinB.
a) the two resistors in series (since no current flows to the input), i.e. 20k?
b) the resistance used in a voltage divider formula, i.e. 10k/10k+10k = 0.5?
c) something else I've overlooked?
Thanks in advance.
EDIT: Added the circuit diagram.