Integration by parts

[chrono210]

How would you go about doing this:

\int64x^2cos(4x)dx

The question specifically asks to integrate it by parts, so I integrated it that way a couple of times and came out with some long mess of sines and cosines, but it's not the right answer.

Thanks.

[genneth]

The approach is correct. The first thing I'd do is a substitution 4x->u just so that the numbers disappear. Then just integrate by parts twice. I suggest you try and post your attempt, so that we can see if it's just a simple algebraic mistake. Like I said, it's going in the right direction.

[chrono210]

Oops, I almost forgot about this. I actually was able to figure it out. Thanks though. :)