# Quick question on tables and integration

1. Mar 6, 2007

### trajan22

ok so here is the problem

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

the table integral I am supposed to use is this
$$\int\frac{du}{\sqrt{u^2-a^2}}=ln(u+\sqrt{u^2-a^2}+C$$

Is it proper to make my u=x^2 and a=2x^(1/2)
I am asking because the solution guide tells me to complete the square, and then proceed to pick the u and a

Last edited by a moderator: Mar 7, 2007
2. Mar 6, 2007

### ZioX

No. a is assumed to be constant.

3. Mar 6, 2007

### trajan22

so if a is any real number i can do this, but if any nonconstant variable is there than I need to find another way? Sorry it has been a while since Ive done this.

4. Mar 7, 2007

### dextercioby

It doesn't matter whether it's "a" or "b", "y", "z",...You integrate wrt "x" and that's all that matters. Don't use tables of integrals, when the integrations can be done explicitely.

5. Mar 7, 2007

### ZioX

Yes, you're right. I was being lazy...constant in this context means a does not depend on x.

Another one of the little nuances in math.

6. Mar 7, 2007

### HallsofIvy

Staff Emeritus
I wouldn't consider the definition of "constant" to be a "little nuance"!