# Proving g(x) is continuous over interval (-∞,-2)

Callumnc1
Homework Statement:
Relevant Equations:
For number 18,

The solution is,

However, should they not write "For ## -∞ < a < -2##" since ##a ≠ -∞## (infinity is not a number)?

Many thanks!

Staff Emeritus
Gold Member
##a## is a number, so can't be negative infinity.

In particular, ##a\in [-\infty,-2)## isn't really a meaningful thing outside of people abusing notation.

Callumnc1
Callumnc1
##a## is a number, so can't be negative infinity.

In particular, ##a\in [-\infty,-2)## isn't really a meaningful thing outside of people abusing notation.

So a better notation than ## a < -2 ## is ## -∞ < a < -2##, correct?

Many thanks!

Staff Emeritus
Gold Member

So a better notation than ## a < -2 ## is ## -∞ < a < -2##, correct?

Many thanks!
No, the ##-\infty<a## notation conveys no additional information.

##a## and ##a>-\infty## are the same thing. You can include the negative infinity, but in no sense is it better here.

Callumnc1
Mentor
However, should they not write "For −∞<a<−2"
##-\infty < a < -2## and ##a \in (-\infty, -2)## are two different notations that say exactly the same thing. In most of the books I've seen, interval notation, as in the 2nd example above, uses a parenthesis to indicate an endpoint that isn't included, and a bracket to indicate that an endpoint is included. I've seen other notations used, but these seem to be a lot rarer.

Also, I don't think any textbook would include "For" in a compound inequality. ##-\infty < a < -2## says everything that needs to be said.

Callumnc1
Callumnc1