# Integration by parts is wrong

1. Sep 27, 2014

### loserspearl

∫(x2 + 7x) cosx dx

If I make v = (x2 + 7x) and du = cosx dx I get
((x2 + 7x) sinx)/2

If I make v = cosx and du = (x2 + 7x) dx I get
((x3/3 + 7x2/2) cosx)/2

using the form X=Y-X to X=Y/2

Neither are correct, what did I do wrong?

2. Sep 27, 2014

### jz92wjaz

Integration by parts:
∫u dv = uv - ∫v du

You are doing the integration by parts incorrectly.

Last edited: Sep 27, 2014
3. Sep 27, 2014

### SteamKing

Staff Emeritus
I recommend you expand the integrand (x2 + 7x) cosx into x2 cos x + 7x cos x, and integrate the two resulting terms.
I would start with integrating the second term of this expansion first.

4. Sep 27, 2014

K thanks