“…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 analytical methods have appeared to obtain the approximate solutions of nonlinear systems, such as the parameter-expansion method [1], the harmonic balance method [2,3,4,6], the energy balance method [7,8], the Hamiltonian approach [10,12], the use of special functions [13,14], the max-min approach [15,16], the variational iteration method [17,18,19,20,21] and homotopy perturbation [22,23,24,25,26,27,28], and others. 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 [29].…”