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$$\int e^{-x} \mbox{d}x = -e^{-x} + C$$ where $C$ is a constant.
Plugging in numbers from zero to very large we can see that $-e^{-x}$ ranges from $-1$ to $0$. By definition the area of the probability distribution has to be one (from zero to one when integrating from zero to infinity) so $C=1$.
This gives a Cumulative Density Function of $$\mbox{CDF}=1-e^{-x} $$