Chapter 2, Problem 2.65P

Use MATLAB to solve for and plot the response of the following models for $0\le t\le 6$ , where the input is $f\left(t\right)=6\cos 3t$ and the initial conditions are zero:

1. $3\ddot{x}+21\stackrel{\dot{}}{x}+30x=f\left(t\right)$
2. $5\ddot{x}+20\stackrel{\dot{}}{x}+65x=f\left(t\right)$
3. $4\ddot{x}+32\stackrel{\dot{}}{x}+60x=3\stackrel{\dot{}}{f}\left(t\right)+2f\left(t\right)$