The solution as given in my book is as follows:

In a parallel resonance circuit, the impedance of the circuit is maximum at the frequency of anti resonance and this frequency is given by

Frequency (f) = [(1/LC) (R^2/L^2)]^(1/2)

I referred many books, but I didnt come across the concept of anti resonance. But when I searched the internet, I found that the presence of non inductive resistance in A.C circuits along with inductor and capacitor, causes the resonance to be obtained at a frequency lower or higher than the frequency given by

f`=[1/(2(pi)][sqrt(LC)]

But I couldnt understand how the above expression was obtained. Could someone please give an idea on how it could it obtained?