用拉氏变换解方程
$$
\left\{\begin{array}{lc}
y^{\prime \prime}-x^{\prime \prime}+x^{\prime}-y=e^t-2, & x(0)=x^{\prime}(0)=0 \\
2 y^{\prime \prime}-x^{\prime \prime}-2 y^{\prime}+x=-t, & y(0)=y^{\prime}(0)=0
\end{array}\right.
$$
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