# Find the integral of xcos5x dx

1. Jan 24, 2008

### fk378

1. The problem statement, all variables and given/known data
Find the integral of xcos5x dx

2. Relevant equations
uv- (integral)vdu

3. The attempt at a solution

I tried to pick u=cos5x and dv=xdx, but the book uses u=x, dv=cos5x dx

Why can't my way work?

2. Jan 24, 2008

### NateTG

It can. Generally, people choose the $u$ and $v$ so that the problem is simplified - the way the book chooses makes the integrand more tractable.