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

8 xcos 5xdx

to beu, with the rest becomingdv.

Since the goal is to produce a simpler integral, we will choose u = 8x.

This means thatdv =----says this is wrong)cos5xdx<