Calculating the Limit as x Tends to Infinity of a Cube Root Expression

  • Thread starter Thread starter sara_87
  • Start date Start date
  • Tags Tags
    Infinity Limit
Click For Summary

Homework Help Overview

The discussion revolves around calculating the limit of a cube root expression as x approaches infinity, specifically the limit of the expression \(\sqrt[3]{x} ((x+1)^{(2/3)}-(x-1)^{(2/3)})\). The subject area involves limits and algebraic manipulation in calculus.

Discussion Character

  • Mixed

Approaches and Questions Raised

  • Participants explore various algebraic identities and manipulations, including factoring and substitutions, to simplify the limit expression. There are attempts to apply the binomial expansion and discussions on the cancellation of terms as x tends to infinity.

Discussion Status

Some participants express uncertainty about their calculations, while others assert confidence in their results. There is a recognition of differing interpretations of the limit, with one participant suggesting a limit of 4/3 based on their reasoning and verification.

Contextual Notes

Participants question the necessity of certain algebraic techniques and the implications of their manipulations on the limit's outcome. There is an ongoing examination of assumptions related to the behavior of terms as x approaches infinity.

sara_87
Messages
748
Reaction score
0

Homework Statement



calculate the limit as x tends to infinity of:

[tex]\sqrt[3]{x}[/tex] ((x+1)[tex]^{(2/3)}[/tex]-(x-1)[tex]^{(2/3)}[/tex])


Homework Equations





The Attempt at a Solution



using the identity: a-b=(a^2-b^2)/(a+b) ; and dividing top and bottom by x,
= lim [tex]\frac{x^{(1/3)}[(1+1/x)^{(4/3)}-(1-1/x)^{(4/3)}}{(1+1/x)^{(2/3)}+(1-1/x)^{(2/3)}}[/tex]
= infinity
is that right? it seems very wrong.
 
Physics news on Phys.org
The limit is 4/3.
Just to make it simpler to write, let u = (x + 1)^(1/3) and v = (x - 1)^(1/3). Then your expression is
[tex]x^{1/3}(u^2 - v^2)[/tex]
[tex]= x^{1/3}(u - v)(u + v)[/tex]
Now multiply by 1 in the form of [tex]\frac{u^2 + uv + v^2}{u^2 + uv + v^2}[/tex]
This will give you [tex]x^{1/3}\frac{(u^3 - v^3)(u + v)}{(u^2 + uv + v^2)}[/tex]
Now undo the substitution and take the limit.
 
thank you
 
so, that means we have to use the binomial expansion inorder to divide by x^1/3 ??
 
sara_87 said:
so, that means we have to use the binomial expansion inorder to divide by x^1/3 ??
No, not at all. You'll have terms with x + 1 and x - 1 to the 1/3 and 2/3 powers. For each of these terms factor as x(1 + 1/x). Depending on the power the original terms are raised to, you'll pull out a factor of x^(1/3) or x^(2/3). In the end, you'll have x to the same power in the numerator as in the denominator, so they cancel.
 
ok, but then i get:
[2x^(2/3){(1+1/x)^(1/3)+(1-1/x)^(1/3)}]/[x^(2/3){(1+1/x)^(2/3)+(1-1/x)^(2/3)+x^-(1/3)(1+1/x)^(1/3)(1-1/x)^(1/3)
after cancellin the x^(2/3) from top and bottom, the last term in the denominator cancels out as x tends to infinity so isn't the limit 4/2 instead of 4/3 beacuse i get 1+1 at the denominator ??
 
My denominator looked like this:
[tex]x^{2/3}[(1 + 1/x)^{2/3} + (1 + 1/x)^{1/3}(1 - 1/x)^{1/3} + (1 - 1/x)^{2/3}][/tex]

The first factor of x^(2/3) cancels with the same factor in the numerator. The part in square brackets approaches 3 as x gets large, so I don't understand what you're saying about the last term cancelling.

I'm certain that the limit is 4/3, both from the work I did and verifying the limit with Excel.
 
oh right, i see. ur absolutely correct and thank you. :)
 

Similar threads

Replies
17
Views
3K
Replies
3
Views
1K
  • · Replies 12 ·
Replies
12
Views
2K
  • · Replies 5 ·
Replies
5
Views
3K
Replies
2
Views
2K
  • · Replies 1 ·
Replies
1
Views
2K
  • · Replies 13 ·
Replies
13
Views
3K
Replies
5
Views
2K
  • · Replies 10 ·
Replies
10
Views
2K
  • · Replies 105 ·
4
Replies
105
Views
11K