# Improper integral

1. Jan 1, 2007

### trajan22

$$\int \frac{dx}{x \sqrt{x^2-4}}$$

there are bounds to this problem but it is irrelevent since my problem is with the integration and not finding the limits.

this integral resembles that of arcsec(x) but im not sure how to deal with the -4.

is there any way to solve this with partial fractions? Or substitution, the squareroot in the denominator is throwing me off for some reason.

sorry for all the recent posts but im trying to teach myself the second part of calculus.

thanks for the help

Last edited: Jan 1, 2007
2. Jan 1, 2007

### cristo

Staff Emeritus
Try the substitution x=2sec(u)

3. Jan 1, 2007

### trajan22

ok but why would i pick that as the substitution, what strategy did you use to choose that as a substitution

4. Jan 1, 2007

### cristo

Staff Emeritus
Well, look at the sqrt in the denominator. We know that sec2x-1=tan2x, and that the derivative of secx is secx*tanx. This hints towards the substitution x=secu. Since we want to be able to factor the 4 out of the sqrt (to enable us to use the trig identity above), we then take x=sqrt(4)secu=2secu.

5. Jan 1, 2007

### trajan22

Sorry I've been sitting here trying to understand this but I'm still confused. How does this substitution help because if we make x=2secu then we have

$$\int \frac{dx}{(2sec(x))\sqrt{(2sec(x))^2-4}}$$

but how is this more manageable than the previous equation.

6. Jan 1, 2007

### d_leet

I changed all your "x"s to "u"s in the integral because that is what they should be after the substitution, but you forgot to substitute for dx after making that substitution. Also it would help to simplify the part under the radical using a trigonometric identity.

7. Jan 2, 2007

### trajan22

Right, sorry I was thinking of something else when i did that substitution. I think I understand this now, thanks for all the input.