The shown circuit is called a difference amplifier, a) Assume that R1 = 15 k and R2 = 50 k, find the differential gain vO vI2−vI1 b) express the output voltage vO in terms of R1, R2, and (vI2−vI1) c) Design the difference amplifier such that the differential gain is 20 and the maximum resistor value is 500k d) What is the output vO if vI2 = vI1