The 4.5v Vref is derived from the nine volt supply from a voltage divider (100k both halves of divider)

The Vref is is applied to the non inverting input of the opamp through a resister. Lets say its a 100k resister, but the crux of my question is here, so maybe its not.

Finally, lets just say the opamp achieves the theoretical infinite input impedance for simplicity's sake. I imagine for our purposes, it might as well be.

Ok, taking the application of Vref to the non inverting through the 100k resistor into account, how does one determine the input resistance of the circuit at that node?