Past exam of the physics A course of the University of Cambridge / 2012 / Paper 1 / Section D / D13 / b / Solution

Let $V_B$ be the non-inverting input voltage. The feedback divider gives $V_B=V_{\mathrm{out}}/K$. With $s=i\omega$, current conservation at this wavefunction node gives $(V_A-V_B)/R_2=s{C}_2{V}_B$, because the amplifier input draws no current. Hence