Given the function $e^ae^{-ba}$
The indefinite integral is $\int e^ae^{-ba} \mathrm da = \int e^{a-ba} \mathrm da$ is $\frac{e^ae^{-ba}}{1-b}$
I get that $\int e^u = e^u\frac{\mathrm du}{\mathrm da}$.
I cannot seem to understand how the $(1-b)$ term ends up in the denominator.
Can someone point out the rule that I am missing that puts the $(1-b)$ in the denominator?