A closed-loop feedback system has the third-order characteristic equation

q(s)=3+as+as+ao = 0, => where the nominal values of the coefficients are a = 3, a = 6, and do = 11. The uncertainty in the coefficients is such that the actual values of the coefficients can lie in the intervals 2 4, 4 a 9, 6a 17. Considering all possible combinations of coefficients in the given intervals, the system is: a. Stable for all combinations of coefficients. b. Unstable for some combinations of coefficients. c. Marginally stable for some combinations of coefficients. d. Unstable for all combinations of coefficients. In Problems 7 and 8, consider the unity feedback system in Figure 12.45, where 2 G(s) (s + 3)