$\frac {2}{L}(-i\hbar)\int_{0}^{n\pi}\sin(u)\cos(u) \, du$
I tried to solve this using integration by parts
and I got
$\frac {2}{L}(-i\hbar)(\sin^2 u +\cos u)|_{0}^{n\pi}$.
But the answer is zero, and according to the above equation, it is zero or some number.
What did I do wrong here?
($n$ is constant positive integer, but not defined.)