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设 $f(x)=\left\{\begin{array}{ll}x, & 0 \leq x \leq 1 \\ 2-x, & 1 < x,\end{array}\right.$ 求 $\int_2^4 f(x-2) \mathrm{e}^{-x} \mathrm{~d} x$;
                        
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