Eqn1:=diff(u(t),t$2)+(1/8)*diff(u(t),t)+4*u(t)=Ft;DE21:=subs(Ft=3*cos(t/4),Eqn1);DE22:=subs(Ft=3*cos(2*t),Eqn1);DE23:=subs(Ft=3*cos(6*t),Eqn1);#Exercise 21:U21:=rhs(dsolve({DE21,u(0)=2,D(u)(0)=0},u(t)));dU21:=diff(U21,t);#Part (a): Plot the forcing and the solution together:plot({3*cos(t/4),U21},t=0..36*Pi);#Part (b): Plot (x(t)=u(t),y(t)=u'(t))- This is a parametric plot.plot([U21,dU21,t=0..36*Pi]);#Exercise 22:U22:=rhs(dsolve({DE22,u(0)=2,D(u)(0)=0},u(t)));dU22:=diff(U22,t);#Part (a): Plot the forcing and the solution together:plot({3*cos(2*t),U22},t=0..20*Pi);#Part (b): Plot (x(t)=u(t),y(t)=u'(t))- This is a parametric plot.plot([U22,dU22,t=0..20*Pi]);#Exercise 23:U23:=rhs(dsolve({DE23,u(0)=2,D(u)(0)=0},u(t)));dU23:=diff(U23,t);#Part (a): Plot the forcing and the solution together:plot({3*cos(6*t),U23},t=0..10*Pi);#Part (b): Plot (x(t)=u(t),y(t)=u'(t))- This is a parametric plot.plot([U23,dU23,t=0..36*Pi]);