# U substitutuion help!

1. Jan 29, 2009

### miller8605

I don't have a way of getting the equation to look nice but it's:

integral of cos(pi/x^11) / x^12

1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

I am having issues even finding what I could use as a U substitution. Any help would be great!

2. Jan 29, 2009

### rootX

Thing causing trouble is cos(pi/x^11) / x^12

3. Jan 29, 2009

### Dick

4. Jan 29, 2009

### miller8605

problem with that is when you take the derivative, you don't have the dx int he problem.

i'm almost certain the U has to equal x^12 as then du would then be 11x^11 and you can divide that by 11 and stick a 1/11 out front. I just don't know how to get du out of that stupid fraction. unless i'm going about it completely wrong and it's not a U substitution and it's a by parts question.

5. Jan 29, 2009

### Dick

If u=pi/x^11 then what do you think is du?

6. Jan 29, 2009

### miller8605

(-11pi*x^10) / x^11

if I used the quotient rule correctly

7. Jan 29, 2009

### Dick

You forgot to square the denominator.

8. Jan 29, 2009

### miller8605

it looks like i did, i forgot the derivate of pi was zero, haha. dumb on my part.

but that doesn't get me anywhere because no where in the original problem is the du. i have to get rid of that 1/x^12 somehow.

9. Jan 29, 2009

### Dick

Simplify (-11pi*x^10) / (x^11)^2.

10. Jan 29, 2009

### miller8605

wow, can't believe i missed that. thanks for all your help, i'll post up my answer here in a couple minutes.

11. Jan 29, 2009

### miller8605

-1/11pi * sin(pi/x^11) + C

12. Jan 29, 2009

### Dick

Looks ok to me.