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Math Help - Identify the verbal description on the interval

  1. #1
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    Identify the verbal description on the interval

    1. (2,4)
    2. (-∞, -5)
    3. (-9, ∞)

    I think I am over-thinking it
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  2. #2
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    Quote Originally Posted by PleaseHelp View Post
    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.
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  3. #3
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    The question is unclear, I know. Would it be as simple as, for #1, "right 2, up 4"?
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  4. #4
    Senior Member eumyang's Avatar
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    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.
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  5. #5
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    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
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  6. #6
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    Quote Originally Posted by PleaseHelp View Post
    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.
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  7. #7
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    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.
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  8. #8
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    Quote Originally Posted by PleaseHelp View Post
    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.
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  9. #9
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    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
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  10. #10
    MHF Contributor undefined's Avatar
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    Quote Originally Posted by PleaseHelp View Post
    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.
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  11. #11
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    Alright, thanks. Have a great day.
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