26. If the system has multiple poles on the jω axis, the system is :

(a) stable

(b) unstable

(c) marginally stable

(d) conditionally stable

27. When the system is excited by an unbounded input and produces an unbounded output :

(a) the system is stable

(b) the system is unstable

(c) the system is conditionally stable

(d) nothing can be said about stability

28. The number of sign changes in the elements of the first column of the Routh array denotes :

(a) the number of zeros of the closed-loop system in the RHP

(b) the number of poles of the closed-loop system in the RHP

(c) the number of open-loop zeros in RHP

(d) the number of open-loop poles in RHP

29. The terms in the first column of the Routh array of the characteristic equation of a certain system are 4, 7, 6, -5, -4, -3. The number of roots of the characteristic equation in the right-half of the s-plane is equal to :

(a) 2

(b) 1

(c) 3

(d) none of these

30. The characteristic equation 1 + G(s)H(s) = 0 of a system is given by s4 + 8s3 +12s2 +8s + K = 0 For the system to remain stable, the value of gain K should be :

(a) zero

(b) 0< K

(c) >11

(d) positives

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