一、如圖一之電路，所有運算放大器均為理想，若R1 = 5 kΩ，R2 = 10 kΩ，R3 = 2.5 kΩ，R4 = 2 kΩ，Ra =Rb =Rc = 10 kΩ，v1 = 500 mV，v2 = 800 mV，v3 = 200 mV，v4 = 100 mV，試推導出電路之輸出電壓vo =？（20 分）