resonance involing an ODE

Hi

my question is

For the ODE below, write down the forcing function $\displaystyle f(t)$that will lead to resonant behaviour $\displaystyle (t\rightarrow\infty)$. Verify your choice by calculating the particular intergral that corresponds to your forcing and examining its limit as $\displaystyle t\rightarrow\infty

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a) $\displaystyle y^{(4)}+5\ddot{y}+4y=f(t)$

my solution to $\displaystyle y^{(4)}+5\ddot{y}+4y=0$ is $\displaystyle y_{c}=Asin(x)+Bcos(x)+Csin(2x)+Dcos(2x)$

thanks