Question

How do you integrate xcos(pix)

Answer 1

Use integration by parts.

Answer 2

Why isn't it -xpisin(pix)?

Answer 3

Because if you differentiate that using the product rule, you will realize that it doesn't end up as \(x\cos(\pi x)\).

Answer 4

The ultimate authority here isn't me, it's the math. Don't be afraid to see what the math does.

Answer 5

That actually helped a lot thanks

Answer 6

If you want assistance on integration by parts, I might be able to help.

Answer 7

Read this
http://tutorial.math.lamar.edu/Classes/CalcII/IntegrationByParts.aspx

Answer 8

You can also practice on my site
http://www.saab.org/calculus.cgi