Determining if the sequence convergers or diverges(IV)

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Discussion Overview

The discussion centers around determining whether the sequence defined by the expression $$\sqrt{n^2 + n} - n$$ converges or diverges as n approaches infinity. Participants explore the limit of the sequence and the methods for evaluating it, including rationalization and the handling of indeterminate forms.

Discussion Character

  • Exploratory
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • Some participants propose that the sequence diverges as n approaches infinity, while others suggest it converges to 1/2.
  • A participant notes that the expression $$\infty - \infty$$ is an indeterminate form, prompting a need for rationalization.
  • One participant provides a detailed rationalization process, leading to the conclusion that the limit is 1/2.
  • Another participant expresses confusion regarding the steps in the rationalization, particularly questioning the treatment of $$n^2$$ and the resulting limit.
  • There is a clarification that $$\sqrt{n^2} = n$$ when n is nonnegative, which is relevant to the limit evaluation.

Areas of Agreement / Disagreement

Participants express differing views on whether the sequence converges or diverges, with no consensus reached on the final outcome. The discussion includes both support for convergence to 1/2 and challenges to the reasoning behind this conclusion.

Contextual Notes

Participants highlight the complexity of handling indeterminate forms and the importance of careful algebraic manipulation. There are unresolved questions about the steps taken in the rationalization process, particularly regarding the treatment of terms as n approaches infinity.

shamieh
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Determining if the sequence converges or diverges, if it converges find the limit

$$\sqrt{n^2 + n} - n$$

Wouldn't this just diverge if n--> infinity ?

I'm not sure what to do here? I can;t use lopitals...Also how would this converge to 1/2 is this a telescoping series?
 
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shamieh said:
Determining if the sequence converges or diverges, if it converges find the limit

$$\sqrt{n^2 + n} - n$$

Wouldn't this just diverge if n--> infinity ?

I'm not sure what to do here? I can;t use lopitals...Also how would this converge to 1/2 is this a telescoping series?

No, $\displaystyle \begin{align*} \infty - \infty \end{align*}$ is an indeterminate form.

You would need to try to rationalise the numerator...

$\displaystyle \begin{align*} \sqrt{n^2 + n} - n &= \frac{\left( \sqrt{n^2 + n} - n \right) \left( \sqrt{n^2 + n} + n \right) }{\sqrt{n^2 + n} + n} \\ &= \frac{n^2 + n - n^2}{\sqrt{n^2 + n} + n} \\ &= \frac{n}{\sqrt{n^2 + n} + n} \\ &= \frac{n}{\sqrt{n^2 \left( 1 + \frac{1}{n} \right) } + n } \\ &= \frac{n}{n\sqrt{1 + \frac{1}{n}} + n} \\ &= \frac{1}{\sqrt{1 + \frac{1}{n}} + 1} \\ &\to \frac{1}{\sqrt{1 + 0} + 1} \textrm{ as } n \to \infty \\ &= \frac{1}{2} \end{align*}$

So the sequence converges to 1/2.

Also this is not a series, so again, don't try to see if this non-existent series is telescopic...
 
Hey thank you so much, but on the 5th step I think I'm confused. Where did the n^2 go? did you pull it out infront of the square root because it is a constant multiplier and if so if you pulled it out n/n^2 would be 1/n not 1/1 right?? Then you would take lim as n -> infty and get 0?
 
shamieh said:
Hey thank you so much, but on the 5th step I think I'm confused. Where did the n^2 go? did you pull it out infront of the square root because it is a constant multiplier and if so if you pulled it out n/n^2 would be 1/n not 1/1 right?? Then you would take lim as n -> infty and get 0?

Surely you can see that $\displaystyle \begin{align*} \sqrt{n^2} = n \end{align*}$ (if n is nonnegative as it is since we are making it go to infinity)...
 
Oh I see.
 

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