I'm doing my homework, and encountered a problem. The problem is the one as attached. It includes a part of solution that I don't understand. I've known that when designing the PI controller, we start from D(s) = (Kp + Ki/S) The condition of the PI controller is, firstly, unity DC gain. So the way I started is setting D(0) = 1, but couldn't proceed anymore cuz there is 1/s in D(s), also it is obviously different from the solution. The second way was setting whole transfer function of the system Gcl(0) = 1, but couldn't find the right solution. According to solution, it approaches Kp and Ki separately. And it uses (G(0)/(1+Kp*G(0))=1), but I can't understand this. Could somebody help me out??