$2\mathrm{.}11.$ Compute the initial conditions such that the response of

$m{x}^{}+kx=F_{0}cost$

oscillates at only one frequency, $().$

$2\mathrm{.}18.$ Consider a spring$-$mass system with zero initial conditions described by

$x^{}(t)+4x(t)=10cos2t,x(0)=0,x^{}(0)=0$

and compute the form of the response of the system.