“…Many scholars have made outstanding contributions and many different methods are obtained such as homotopy perturbation method, [1][2][3] variational approach, [4][5][6][7][8][9] variational iteration method, [10][11][12][13][14][15][16] He's frequency formulation, [17][18][19] Hamiltonian approach, 20 Taylor series method, 21 and so on. [22][23][24] The well-known Duffing oscillator equation was named after a German electrical engineer Georg Duffing who first proposed the equation in 1918, 25 and then, it is developed into different forms to describe many physical, mechanical engineering, circuits and biological processes in various areas of science. [26][27][28] Thus, the study of the Duffing oscillator equation is important.…”