“…In last decades, some analytical approximate methods and their modifications were suggested by researchers for solving nonlinear problems such as Adomian decomposition method [6,7,8], variational iteration method (VIM) [8,9,10,13], differential transform method [8,11], homotopy perturbation method [12, 13,14], harmonic balance method [15], incremental harmonic balance method [16], Newton-harmonic balance method [17], variational approach method [18], amplitude-frequency formulation [19][20][21], energy balance method [20], max-min approach [21]. Variety of methods to solve nonlinear problems provide more reliable results for the modal parameter on the vibration period.…”