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

In summary, the conversation involves calculating a limit as x tends to infinity, using an algebraic manipulation and substitution to simplify the expression and ultimately reaching the conclusion that the limit is 4/3.
  • #1
sara_87
763
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.
 
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  • #2
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.
 
  • #3
thank you
 
  • #4
so, that means we have to use the binomial expansion inorder to divide by x^1/3 ??
 
  • #5
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.
 
  • #6
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 ??
 
  • #7
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.
 
  • #8
oh right, i see. ur absolutely correct and thank you. :)
 

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

1. What is the definition of "limit as x tends to infinity"?

The limit as x tends to infinity is a mathematical concept that represents the behavior of a function as the input value (x) approaches infinity. It is denoted as lim f(x) as x approaches infinity and is used to determine the value that a function approaches as its input value grows without bound.

2. How is the limit as x tends to infinity calculated?

The limit as x tends to infinity is calculated by evaluating the function at increasingly larger values of x. As x gets closer and closer to infinity, the output value of the function will approach a specific value. This value is the limit as x tends to infinity.

3. What does it mean if the limit as x tends to infinity does not exist?

If the limit as x tends to infinity does not exist, it means that the function does not approach a specific value as x gets larger and larger. This could be due to the function oscillating between different values or approaching different values from different directions.

4. What are some real-world applications of the limit as x tends to infinity?

The limit as x tends to infinity has many applications in physics, engineering, and economics. It is used to model the behavior of systems such as population growth, radioactive decay, and fluid flow. It is also used to analyze the stability and convergence of numerical algorithms.

5. How does the limit as x tends to infinity relate to the concept of infinity?

The limit as x tends to infinity is used to describe the behavior of a function as its input value approaches infinity. It is not the same as infinity itself, but rather a way to describe how a function behaves as it approaches infinity. In some cases, the limit as x tends to infinity may approach infinity, but this is not always the case.

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