# Need some help with this integral

1. Apr 12, 2006

### Grogs

I ran into the following (general form) integral today while working on an engineering problem:

$$\int \frac{dx}{x (a \sqrt{x} +b)}$$

I ran it through 'integrals.com' and came up with the following solution:

$$\frac {\ln{x} - 2 \ln{(a \sqrt{x} + b)}}{b}$$

which is fine for the purpose of solving the problem, but I'd like to be able to work it by hand. I tried integration by parts, but any choice of u and dv I could think of only made it messier. I also couldn't think of an appropriate u substitution.

Grogs

Last edited: Apr 12, 2006
2. Apr 12, 2006

### arildno

Try $u=\sqrt{x}$
With the use of partial fractions decomposition, the result easily follows.

3. Apr 12, 2006

### Grogs

Thanks arildno.

Partial fractions is one of those skills I relearn once a year or so when I need to do a LaPlace Transform and then quickly forget.