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f^{\prime}(t)=\frac{\mathbf{d}}{d t}(f(t))=\frac{\mathbf{d}}{d t}\left(40 \cdot e^{\mathrm{t} / 5}\right)=40 \cdot \mathrm{e}^{\mathrm{t} / 5} \frac{\mathbf{d}}{\mathbf{d} t}(\mathrm{t} / 5)