Simple question, but marked me wrong. Am I forgetting something?
To use the integration-by-parts formula
integral udv= uv - integral vdu
we must choose one part of
8x cos 5x dx
to be u, with the rest becoming dv.
Since the goal is to produce a simpler integral, we will choose u = 8x.
This means that dv = cos5xdx <----says this is wrong)