“…Recently, some of them were modified (Cheung et al, 1991) to investigate strong nonlinear problems. Homotopy perturbation (Belendez, 2007;Ganji and Sadighi, 2006;Belendez et al, 2008;Ozis and Yildirim, 2007), iteration method (Haque et al, 2013;Jamshidi and Ganji, 2010;Lim et al, 2006;Baghani et al, 2012;Rafei et al, 2007) are useful for obtaining approximate periodic solution with large amplitude of oscillations; however, they are applicable only for odd nonlinearity problems. Harmonic balance method (Mickens, 1986;Lim et al, 2005;Belendez et al, 2006;Wu et al, 2006;Mickens, 2007;Alam et al, 2007;Belendez et al, 2009;Lai et al, 2009;Hosen et al, 2012) is a powerful method in which truncated Fourier series is used.…”