Is My Approach to Proving lim (x^3+2x^2) = 1 Using ε/δ Definition Correct?

  • Thread starter cwz
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Sorry for the bother.In summary, the conversation discusses proving the limit of (x^3+2x^2) as x approaches -1 using the epsilon-delta definition. The person has made it to the step where δ(δ^2-δ-1) ≤ δ ≤ ε and has chosen ε=min(2,ε) but is unsure if it is correct. Another person points out that the limit does not equal 1 but the first person argues that at x=-1, the equation results in 1 and therefore should be continuous.
  • #1
cwz
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Homework Statement


Prove using ε/δ definition,

lim x tends to -1 (x^3+2x^2) = 1


Homework Equations





The Attempt at a Solution


I have done to the step where δ(δ^2-δ-1) ≤ δ ≤ ε

so i choose ε=min(2,ε)

Not sure whether I am correct or not
 
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  • #2
I hope you realize that
$$\lim_{x \to -1} (x^3+2x^2) \ne 1$$ so you're going to have a tough time proving it. In any case, you need to show more of your work. We can't see your paper or read your mind to see what you actually did.
 
  • #3
vela said:
i hope you realize that
$$\lim_{x \to -1} (x^3+2x^2) \ne 1$$

[itex](-1)^3 + 2(-1)^2 = -1 + 2 = 1[/itex]. Last time I checked, [itex]x^3 + 2x^2[/itex] was continuous everywhere.
 
  • #4
Well, now I feel like an idiot. :wink: And I checked it over and over and kept getting -1.
 

What is the ε/δ definition?

The ε/δ definition, also known as the epsilon-delta definition, is a method used to prove the limit of a function. It involves using two variables, epsilon (ε) and delta (δ), to show that as the input approaches a certain value, the output of the function also approaches a specific value.

Why is the ε/δ definition important in mathematics?

The ε/δ definition is important because it provides a rigorous and precise way to determine the limit of a function. It allows for a more formal and logical approach to proving limits, rather than relying on intuition or graphical interpretations.

How do you use the ε/δ definition to prove a limit?

To prove a limit using the ε/δ definition, you first choose a value for epsilon (ε) and then find a corresponding value for delta (δ) that satisfies the definition. This involves manipulating the function and setting constraints on the values of ε and δ to show that the limit exists and is equal to the desired value.

What are some common challenges when using the ε/δ definition?

One common challenge when using the ε/δ definition is choosing the correct values for ε and δ. It requires a lot of algebraic manipulation and can be time-consuming. Another challenge is understanding the concept of a limit and how it relates to the specific function being analyzed.

How does the ε/δ definition relate to real-world applications?

The ε/δ definition is used in many real-world applications, such as physics, engineering, and economics. It allows us to determine the behavior of a system as it approaches a certain value, which is crucial in understanding and predicting real-world phenomena. For example, the concept of limits is used in calculus to analyze motion and change in physical systems.

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