The amplitude of vibration of a particle is given by am=(a0)(aω2−bω+c) where a0,a,b and c are positive. The condition for a single resonant frequency is
b2=4ac
For resonance amplitude must be maximum which is possible onlu.
When the denominator of expression is zero i.e. aω2−bω+c=0⇒ω=+b±√b2−4ac2a
For a single resonant frequency , b2=4ac .