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**Johnathon bresley****Guest**

How do i prove this using delta epsilon-

Lim x^2-1=3

x-->2

(please use d and e for delta and epsilon)

**noelevans****Member**- Registered: 2012-07-20
- Posts: 236

Hi Johnathon,

Use the definition:

limf(x)=L means for each e>0 there is a d>0 such that |x-a|<d ==> |f(x)-L|<e; that is,

x-->a

2 2

lim(x - 1)=3 means for each e>0 there is a d>0 such that |x-2|<d ==> |(x - 1) -3|<e.

x-->2

Now choose d in terms of e. Example: let d=g(e) but be specific like d=sqrt(e). You may have to

get creative. Then you need to "transform" the |x-2|<g(e) into |x^2-1-3|<e;that is, you need

to show that |x-2|<d implies that |x^2 - 4|<e.

That's the general procedure. But there are many functions that finding an appropriate d and

doing the "transformation" is nigh unto impossible. That's why they prove that products, sums,

differences, and quotients (excluding where the denominator may be or approach zero) are

continuous. This makes all polynomials continuous. Then x^2-1 is continuous since it is a product

and a difference of the continuous functions x and 1..

Then the nice little theorem that says if a function is continuous at a then the limit as x-->a is f(a).

makes the limit here as x-->2 turn out to be f(2)=2^2 - 1 = 3.

Good luck!

By the way, what would it mean to say that L is NOT the limit of f(x) as x approches a?

*Last edited by noelevans (2012-08-16 08:16:43)*

Writing "pretty" math (two dimensional) is easier to read and grasp than LaTex (one dimensional).

LaTex is like painting on many strips of paper and then stacking them to see what picture they make.

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