Hi,
if the source is low impedance, then the voltage on U1/pin 2 only depends on input signal and not on R9 current.
and the other way with it´s series impedance of more than 1.5MOhms it won´t significally change the output signal with an impedance of abot 1500 Ohms.
So neither the right side of R9 can change the OUT put signal, nor the left side of R9 can change the input signal. --> no function at all.
******
But if input impedance is high (maybe100k) then the current through R9 will change the signal into U1/pin 2.
--> You need the input impedance (source impedance) to calculate the circuit.
Klaus