“…It is very important in applications to have a version of the frequency (or period) to have a better understanding of the phenomena modeled through differential equations that contain terms with high nonlinearities, and a simple mathematical method is very useful for practical applications. Recently, many different methods have been developed to obtain the approximate solutions of nonlinear systems including the harmonic balance method [1,2,3,4], the energy balance method [5,6], the Hamiltonian approach [7,8], the use of special functions [9,10], the max-min approach [11,12], the variational iteration method [13,14,15,16,17] and homotopy perturbation [18,19,20,21,22,23,24]. An excellent study, in which many of these techniques can be found in detail to solve nonlinear problems of oscillatory type can be seen in [25].…”