How Do You Solve a Limit Involving Infinity in Polynomial Functions?

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Discussion Overview

The discussion revolves around solving a limit involving infinity in polynomial functions, specifically the limit of the expression (3x + √(9x² - x)) as x approaches negative infinity. Participants explore various approaches to evaluate this limit, including substitution and algebraic manipulation.

Discussion Character

  • Homework-related
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant expresses difficulty in evaluating the limit and initially concludes it to be -∞ based on substitution.
  • Another participant suggests rewriting the limit using absolute values and hints at a different approach to simplify the expression.
  • A participant questions the notation used in the hint, suggesting that it should reflect the negative approach of x towards -∞.
  • There is a correction regarding the expression's manipulation, with participants discussing the need to expand the square root and perform cancellations.
  • One participant challenges the conclusion of -∞, indicating that the limit may not be as stated and prompting a reevaluation of the expression as x approaches -∞.
  • A later post suggests multiplying by a conjugate to simplify the limit further, indicating an alternative method to approach the problem.

Areas of Agreement / Disagreement

Participants do not reach a consensus on the limit's value, with multiple competing views and approaches presented throughout the discussion.

Contextual Notes

There are unresolved mathematical steps and assumptions regarding the manipulation of the limit expression, particularly concerning the treatment of infinity and the square root function.

mohamedibr752
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Hi, I'm in Engineering Foundation.
I'm stuck in one limit question.

Find the limit :_
**********________
Lim (3x + V 9x^2 - x )
x-> -infinity

by substitution it gives ( inf - inf )
I tried to solve it and get -inf

Can anyone help me please ?
 
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Welcome to PF!

Hi mohamedibr752! Welcome to PF! :smile:

(have a square-root: √ and an infinity: ∞ :wink:)
mohamedibr752 said:
Hi, I'm in Engineering Foundation.
I'm stuck in one limit question.

Find the limit :_
**********________
Lim (3x + V 9x^2 - x )
x-> -infinity

by substitution it gives ( inf - inf )
I tried to solve it and get -inf

Can anyone help me please ?


(no need to shout! :redface:)

Hint: it's the same as limx->-∞ 3x + 3|x|√(1 - 1/9x)) :wink:
 
Last edited:
Thank you for your help .. I get it now ..
but why did you write it 3x(1 + ...
I think it is 3x(1 - ...
because the x is approaching -∞ so as to remove absolute 3x we have to put - to make sure that the 3x is positive ??

The answer I get is -∞.

Thank you
 
Hi mohamedibr752! :smile:
mohamedibr752 said:
but why did you write it 3x(1 + ...
I think it is 3x(1 - ...

I goofed! :redface:

I did correct it before you answered, but I think you missed the change. :wink:
The answer I get is -∞.

No, try again … expand the square-root and do a bit of cancelling. :smile:
 
mohamedibr752 said:
The answer I get is -∞.

Wait, what? No the answer isn't -\infty.

Look again at 3x+3|x|\sqrt{1-\frac{1}{9x}} as the limit approaches -\infty. It is definitely not what you stated.
 
mohamedibr752 said:
Hi, I'm in Engineering Foundation.
I'm stuck in one limit question.

Find the limit :_
**********________
Lim (3x + V 9x^2 - x )
x-> -infinity

by substitution it gives ( inf - inf )
I tried to solve it and get -inf

Can anyone help me please ?


Multiply by \frac{3x -\sqrt{9x^2-x}}{3x-\sqrt{9x^2-x}} ... and then divide by x,


then take the limit
 

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