Consider
\begin{align}
y = \int \sin(e^ x) \ dx
\end{align}
How do I integrate that, I know the result is $y'=\sin(e^x)$ but why? 

So, I can't do it straight away, so I've got to use either substitution or integration by parts, as it is a composition I would say substitution but I don't see any relation between $\cos$ and $e^x$ ...any detailed help about how to proceed? thanks!