Q.
The amplitude of velocity of a particle is given by, Vm=V0/(aω2−bω+c) where V0,a,b and c are positive : The condition for a single resonant frequency is
Vm=V0/(aω2−bω+c)
If there is a single resonant frequency, then this equation should be satisfied for only one that particular resonant frequency, hence aω2−bω+c=0 will have equal roots therefore; D=0⇒(−b)2−4ac=0⇒b2=4ac