Substition of these equations in (6) yiedts
\[
\begin{array}{l}
A e^{t}+B e^{3 t}+C e^{-2 t}+E+A e^{t}+3 B e^{3 t}-2 C e^{-2 t}=4 e^{t}-8 c_{1} e^{2 t}-3 c_{2} e^{-2 t}-5 \\
2 A e^{t}+4 B e^{3 t}-C e^{-2 t}+E=4 e^{t}-8 c_{1} e^{3 t}-3 C_{2} e^{-2 t}-5
\end{array}
\]
Comparing cefficients we find that
- \( e^{t}: 2 A=4 \Rightarrow A=2 \)
- \( e^{3 t}: 4 B=-8 c_{1} \Rightarrow B=-2 c_{1} \)
- \( e^{-2 t}:-C=-3 c_{1} \Rightarrow C=3 c_{2} \)
+ Const. \( E=-5 \)
\[
\Rightarrow y_{P, I}(t)=2 e^{t}-2 c_{1} e^{3 t}+3 c_{2} e^{-2 t}-5
\]
Therefora,
\[
\begin{aligned}
y(t) & =y_{C, F}(t)+y_{P, I}(t) \\
& =C_{3} e^{-t}+2 e^{t}-2 C_{1} e^{3 t}+3 C_{2} e^{-2 t}-5
\end{aligned}
\]
Tho solution therefore contains is arbitrary constants \( C_{1}, C_{2} \) and \( C_{3} \) wheress the determinant
\[
\operatorname{dat}\left|\begin{array}{ll}
D+5 & D+1 \\
D+11 & 2 D+1
\end{array}\right|=D^{2}-D-6
\]
is of the seand order. Substitution of the solution \( (x(t), y(t)) \) inte (2) yields \( C_{3}=0 \). Consequently a general solution of the systan is given by
\[
\begin{array}{l}
x(t)=c_{1} e^{3 t}+c_{2} e^{-2 t}-\frac{1}{2} e^{t}+1 \\
y(t)=2 e^{t}-2 c_{1} e^{3 t}+3 c_{2} e^{2 t}-5
\end{array}
\]
with anly 2 arbitrary constants \( C_{1} \) and \( C_{2} \).
2.1. Solve the system: \( -2 \dot{x}+\ddot{y}+y=t^{2} \)
\[
\dot{x}-2 x+2 \dot{y}-y=3 t^{2}+3,
\]
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by using the elimination methed (qoerator method). Eliminate a first.
Edution \( D \)-gerator form: \( -2 D[x]+D^{2}[y]+[y]=-t^{2} \)
\[
\begin{aligned}
& \Rightarrow-2 D[x]+\left(D^{2}+1\right)[y]=-t^{2} \ldots(1 \\
\text { and } & D[x]-2[x]+2 D[y]-[y]=3 t^{2}+3 \\
\Rightarrow & (D-2)[x]+(2 D-1)[y]=8 t^{2}+3 \ldots
\end{aligned}
\]
Eliminating \( x \) first we find:
\[
\begin{aligned}
\cdot(D-2)[(1)] & :(D-2)(-2 D[x])+(D-2)\left(D^{2}+1\right)[y] \\
\Rightarrow & =\left(D-2 D^{2}+4 D\right)[x]+\left(D^{2}\right) \\
\cdot-2 D[(2)] & :-2 D(D-2)[x]-2 D(2 D-1)[y]=-2 D\left(3 t^{2}+3\right) \\
\Rightarrow & \left(-2 D^{2}+4 D\right)[x]+\left(-4 D^{2}+2 D\right)[y]=-12 t
\end{aligned}
\]
Naw,