# Integration of 3 lnx/x

1. Mar 3, 2008

### Mitchtwitchita

Can anybody help me out with this?

3lnx/x du = ( ? ) dx. What is the question mark? Leave no spaces in your answer.

I thought u was 3lnx, which would be make it 3/x dx. Apparently not. Can anybody show me the right way of doing this sort of problem please?

2. Mar 3, 2008

### qspeechc

Are you trying to solve for u? If so:

(x/3lnx)dx = du

You can integrate both sides, the left by integration by parts, I think will work.

3. Mar 3, 2008

### HallsofIvy

That really makes no sense without knowing how u depends on x. Why did you "think u was 3ln(x)"? Are you leaving out part of the problem?

4. Mar 3, 2008

### Mitchtwitchita

Sorry, I should have been more clear. I just copied and pasted the problem. The question was 3lnx/x, and I just needed to find the first part of the problem. The professor wanted us to "find u, and take it one more step." I thought that step was differentiating u. All I really need is what would be in the brackets with respect to dx.

5. Mar 3, 2008

### HallsofIvy

Then knowing that the derivative of ln(x)= 1/x should help you! 3ln(x)/x dx= 3(ln x)(dx/x). I would incline toward setting u= ln x. What is du now?

6. Mar 3, 2008

1/x?

7. Mar 4, 2008

### johndoe

should be 1/x dx precise.