# limit to infinity

• Feb 9th 2010, 09:46 AM
ryan18
limit to infinity
Could someone please show me how to workout problems of these types:

Find the limit. (If you need to use -http://www.webassign.net/images/infinity.gif or http://www.webassign.net/images/infinity.gif, enter -INFINITY or INFINITY.)
http://www.webassign.net/cgi-bin/sym...20-%206%20x%29

• Feb 9th 2010, 09:51 AM
felper
Quote:

Originally Posted by ryan18
Could someone please show me how to workout problems of these types:

Find the limit. (If you need to use -http://www.webassign.net/images/infinity.gif or http://www.webassign.net/images/infinity.gif, enter -INFINITY or INFINITY.)
http://www.webassign.net/cgi-bin/sym...20-%206%20x%29

Try multiplying by $\frac{\sqrt{36x^2+x}+6x}{\sqrt{36x^2+x}+6x}$ and then dividing by x up and down (i think that it have to works)
• Feb 9th 2010, 10:00 AM
ryan18
How did you get the $\frac{\sqrt{36x^2+x}+36x^2}{\sqrt{36x^2+x}+36x^2}$?
• Feb 9th 2010, 10:15 AM
felper
Quote:

Originally Posted by ryan18
How did you get the $\frac{\sqrt{36x^2+x}+36x^2}{\sqrt{36x^2+x}+36x^2}$?

Oh, i did a little mistake. I've corrected it. Now that it's correct, with that factor, you can eliminate the root. See what happens:

$\lim_{x\to\infty}\sqrt{36x^2+x}-6x \cdot \frac{\sqrt{36x^2+x}+6x}{\sqrt{36x^2+x}+6x}=\lim_{ x\to\infty}\frac{x}{\sqrt{36x^2+x}+6x}=\lim_{x\to\ infty}\frac{1}{\sqrt{36+\frac{1}{x}}+6}$

Now you can conclude.
• Feb 9th 2010, 10:23 AM
ryan18
How did you go from $
=\lim_{x\to\infty}\frac{x}{\sqrt{36x^2+x}+6x}=\lim _{x\to\infty}\frac{1}{\sqrt{36+\frac{1}{x}}+6}
$

[/tex]?? It looks like in some places you multiplied out by $\frac{1}{x}$ and other parts you multiplied out by $\frac{1}{x^2}$
• Feb 9th 2010, 10:29 AM
felper
Quote:

Originally Posted by ryan18
How did you go from $
=\lim_{x\to\infty}\frac{x}{\sqrt{36x^2+x}+6x}=\lim _{x\to\infty}\frac{1}{\sqrt{36+\frac{1}{x}}+6}
$

[/tex]?? It looks like in some places you multiplied out by $\frac{1}{x}$ and other parts you multiplied out by $\frac{1}{x^2}$

I've multiplied by 1/x both sides. $\frac{1}{x}(\sqrt{36x^2+x}+6x)=\frac{1}{x}(\sqrt{3 6x^2+x})+6=\frac{1}{\sqrt{x^2}}(\sqrt{36x^2+x})+6= \sqrt{36+\frac{1}{x}}$
• Feb 9th 2010, 10:36 AM
ryan18
Ok I see now, so the denominator would look like $\sqrt{36+\frac{1}{x}}+6$ correct? and the $\frac{1}{x}$ would tend to 0. Leaving $\sqrt{36}+6=12$? Making the final answer to be $\frac{1}{12}$??
• Feb 9th 2010, 10:43 AM
felper
Yep
• Feb 9th 2010, 10:46 AM
ryan18
Awesome thank you so much for putting up with my multiple questions!