# Homework Help: Problem evaluating a limit as x to infinity algebraically

1. Feb 15, 2009

### fedaykin

1. My problem is such:

Find the limit of $$\lim_{x \rightarrow \infty} \sqrt{9x^2+x} -3x$$

2. No relevant equations

3. I multiplied $$\frac{\sqrt{9x^2+x} -3x}{1} * \frac{\sqrt{9x^2+x} +3x}{\sqrt{9x^2+x} +3x} = \frac{x}{\sqrt{9x^2+x} +3x}$$

I am now quite confused as to where to go from here. My teacher will accept tabled values, but I wish to prove my answer. I would greatly appreciate any help.

I did attempt to divide by the highest power in the denominator, but all that got me was a mess:

$$\frac{1}{\frac{\sqrt{9x^2+x}}{x} +3}$$

Ok, I'm not certain this is valid, but...:

I'll factor out an x under the radical, then attempt to simplify

$$\frac{1}{\frac{\sqrt{9x^2*(1+\frac}{x}{9x^2}{x} +3}$$

Hmm... I'm having trouble with tex and that.

Last edited: Feb 15, 2009
2. Feb 15, 2009

### Office_Shredder

Staff Emeritus
If you pull an x out of the square root you get

$$\frac{x}{x ( \sqrt{9 + 1/x} + 3)} = \frac{1}{ \sqrt{9+1/x} + 3}$$

which you should be able to do

3. Feb 15, 2009

### fedaykin

Thank you so very much.