Problem: Given \[ L(s)=\frac {s}{(s+4)(s+5)}\] as the open loop transfer function, how to ﬁnd \(G(s)\), the closed loop transfer function for a unity feedback?
Mathematica
Clear["Global`*"]; sys = TransferFunctionModel[ s/((s+4)(s+5)),s]


closedLoop = SystemsModelFeedbackConnect[sys];


The system wrapper can be removed in order to obtain the rational polynomial expression as follows
Matlab
clear all; close all; s=tf('s'); sys=s/((s+4)*(s+5)); closedloop=feedback(sys,1)

Transfer function: s  s^2 + 10 s + 20 