Solve the equation of motion \(\ddot{x}+0.5 \dot{x}+x+1.2 x^{3}=1.8 \cos 0.4 t\), using the Runge-Kutta method with \(\Delta t=0.05, t_{\max }=5.0\), and \(x_{0}=\dot{x}_{0}=0\). Plot the variation of \(x\) with \(t\). Use Program18.m for the solution.