4. Derive the closed-loop transfer function between r₁(s) and y,(s) for the follow- ing control block diagram (this is known as a cascade control system). Hint: Find a single block to represent the "inner" feedback loop. 2 e2 12 gcl gc2 8p2 P + + y m2 8 m2 y ml gml y 1 y 2 gpl How would you check the stability of the closed-loop system?