13.11. For the control system in Fig. P13-11, the characteristic equation is $4 + 4s? +682 + 4s + (1 + k) = 0 (a) Determine the value of K above which the system is unstable. (b) Determine the value of K for which two of the roots are on the imaginary axis, and determine the values of these imaginary roots and the remaining two roots. K +1