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Limits from above and below?

Poly

Member
Nov 26, 2012
32
Could someone explain what things like [tex]\displaystyle \lim_{x \to 3^+}\frac{1}{x}[/tex] and [tex]\displaystyle \lim_{x \to 3^-}\frac{1}{x}[/tex] are supposed to be?

I googled and it gave me epsilon delta stuff. I'm not smart enough for that. (Doh)

I guess I'm asking if anyone could give a dumbed down explanation of sorts. :D
 

dwsmith

Well-known member
Feb 1, 2012
1,673
Could someone explain what things like [tex]\displaystyle \lim_{x \to 3^+}\frac{1}{x}[/tex] and [tex]\displaystyle \lim_{x \to 3^-}\frac{1}{x}[/tex] are supposed to be?

I googled and it gave me epsilon delta stuff. I'm not smart enough for that. (Doh)

I guess I'm asking if anyone could give a dumbed down explanation of sorts. :D
Plus means approaching 3 from the right so 3.1, 3.01, 3.001 etc
I bet you can guess what minus means.
 

Poly

Member
Nov 26, 2012
32
Why did you start with 3.1?
 

dwsmith

Well-known member
Feb 1, 2012
1,673
Why did you start with 3.1?
It isnt really a starting point. I was just emphasizing approaching from the right. We could start at 100 but the goal is to approach 3 and get really close to 3. Really close as in 3.000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001
 

Poly

Member
Nov 26, 2012
32
Thanks. Did I read that sometimes [tex]\lim_{x \to a^{+}}f(x)[/tex] and [tex]\lim_{x \to a^{-}}f(x)[/tex] can be different? How's that?
 

dwsmith

Well-known member
Feb 1, 2012
1,673
Thanks. Did I read that sometimes [tex]\lim_{x \to a^{+}}f(x)[/tex] and [tex]\lim_{x \to a^{-}}f(x)[/tex] can be different? How's that?
That is true. You can have jump discontinuities or one side going to neg inf and the other to pos inf.
Look at the Heaviside Function has an example
 

dwsmith

Well-known member
Feb 1, 2012
1,673
Here is an example you can try:
$$
\lim_{x\to 1}\frac{x^2 - 2x - 3}{x-1}
$$
You need to check the left and right limit.