The necessary condition for the stability of the linear system is that all the coefficients of characteristic equation 1+G(s)H(s) =0, be real and have the :
(a) Positive sign
(b) Negative sign
(c) Same sign
(d) Both positive and negative
I have been asked this question in an online interview.
My question is from Routh-Hurwitz Stability Criterion topic in chapter Stability and Algebraic Criteria of Control Systems