Identify the verbal description on the interval

**PleaseHelp** · July 6th 2010, 10:26 AM

1. (2,4)
2. (-∞, -5)
3. (-9, ∞)

I think I am over-thinking it

**undefined** · July 6th 2010, 10:32 AM

Originally Posted by PleaseHelp

1. (2,4)
2. (-∞, -5)
3. (-9, ∞)

I think I am over-thinking it

Well it's not clear what you're being asked to find, but if you just need to verbally describe the interval, I'd go with:

1. $x \in (2,4)$ means x is a real number between 2 and 4 exclusive.
2. ...a real number less than -5.
3. ...a real number greater than -9.

**PleaseHelp** · July 6th 2010, 10:34 AM

The question is unclear, I know. Would it be as simple as, for #1, "right 2, up 4"?

**eumyang** · July 6th 2010, 10:38 AM

These are not ordered pairs! Interval notation provide just another way to indicate a range of numbers. Brackets mean "including", and parentheses mean "not including". You can also rewrite intervals as compound inequalities.

Take a look:
$(a, b)$ is $a < x < b$
$[a, b]$ is $a \le x \le b$
$(a, b]$ is $a < x \le b$
$[a, b)$ is $a \le x < b$
$(-\infty, c)$ is $x < c$
$(-\infty, c]$ is $x \le c$
$(c, \infty)$ is $x > c$
$[c, \infty)$ is $x \ge c$

We never use brackets next to infinity or negative infinity.

Now try rewriting your intervals as inequalities.

**PleaseHelp** · July 6th 2010, 10:40 AM

Thats great. Thank you. Also, it says in some other questions like (exactly like the piece of paper):

#4. 9,10)
#5. -2,1
#6. (-∞, -5

**undefined** · July 6th 2010, 10:49 AM

Originally Posted by PleaseHelp

Thats great. Thank you. Also, it says in some other questions like (exactly like the piece of paper):

#4. 9,10)
#5. -2,1
#6. (-∞, -5

These are not well-formed intervals.

**PleaseHelp** · July 6th 2010, 10:51 AM

I know. Would it be a typo? Because I am not understanding why the missing brackets in 4 and 6. And 5 isn't even an interval.

**undefined** · July 6th 2010, 10:55 AM

Originally Posted by PleaseHelp

I know. Would it be a typo? Because I am not understanding why the missing brackets in 4 and 6. And 5 isn't even an interval.

Without being able to see your piece of paper, I would say either a typo or a bad photocopy/print-out.

**PleaseHelp** · July 6th 2010, 11:00 AM

I don't have a scanner but I send you to the link of the piece of paper. Its a PDF file. I am 100% it is not a bad photocopy/print-out.

Here is the link. Page 3.

http://susd.desertmountain.schoolfus...682007b4f66e36

**undefined** · July 6th 2010, 11:09 AM

Originally Posted by PleaseHelp

I don't have a scanner but I send you to the link of the piece of paper. Its a PDF file. I am 100% it is not a bad photocopy/print-out.

Here is the link. Page 3.

http://susd.desertmountain.schoolfus...682007b4f66e36

Nothing against your teachers, but I think they should have done a better job of proofreading. My guess is that it's supposed to be

[9,10)
[-2,1]
(-∞, -5]

and the [ ] weren't displayed properly, for whatever reason.

**PleaseHelp** · July 6th 2010, 11:14 AM

Alright, thanks. Have a great day.

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