The open-loop gain of the operational amplifier shown in Fig. 3 is 5. Assume that the amplifier has infinite input resistance, please answer the following questions. (a). Please express the voltage at inverting input (V-) in terms of Vo. (4 points) (b). Please express the voltage at inverting input (V-) in terms of Vi. (6 points) (c). Find the closed-loop gain which is defined as V./V of the circuit. (4 points) (d). Derive the input resistance (Ri) of the overall circuit. (6 points) Fig. 3 Vi R 1 2 + 20k Vo