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Please Explain - Inequalities Problem

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Please Explain - Inequalities Problem Post Tue Jun 26, 2012 9:51 pm
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  • Lap #[LAPCOUNT] ([LAPTIME])
    -------------------------------
    -5 -4 -3 -2 -1 0 1 2 3 4 5

    Which of the following inequalities is an algebraic expression for the red part of the number line above?

    A) |x| <= 3

    B) |x| <= 5

    C) |x-2| <= 3

    D) |x-1| <= 4

    E) |x+1| <= 4

    Please explain.

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    Post Tue Jun 26, 2012 11:56 pm

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    Post Wed Jun 27, 2012 12:06 am
    Jeevanantham wrote:
    -------------------------------
    -5 -4 -3 -2 -1 0 1 2 3 4 5

    Which of the following inequalities is an algebraic expression for the red part of the number line above?
    The extreme points are -5 and 3
    The distance between -5 and 3 on the number line is (3 - (-5)) = 8
    The middle point of the range is -1

    Hence, the expression for the red part is |x - (-1)| ≤ 8/2 ---> |x + 1| ≤ 4

    The correct answer is D.

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    Post Wed Jun 27, 2012 4:15 am
    Jeevanantham wrote:
    -5 -4 -3 -2 -1 0 1 2 3

    Which of the following inequalities is an algebraic expression for the red part of the number line above?

    A) |x| <= 3

    B) |x| <= 5

    C) |x-2| <= 3

    D) |x-1| <= 4

    E) |x+1| <= 4

    Please explain.
    }x-y| = the DISTANCE between x and y

    The midpoint of the range = -1.
    x can be any value from -5 (4 places BELOW -1) to 3 (4 places ABOVE -1).
    In other words: the distance between x and -1 is less than or equal to 4.
    In math terms:
    |x-(-1)| ≤ 4.
    |x+1| ≤ 4.

    The correct answer is E.

    An alternate approach is to plug numbers into the answer choices.
    The correct answer choice must work for EVERY value between -5 and 3, inclusive -- and for no values outside this range.
    Since |x| = the distance between x and 0, start with the value furthest from 0:
    Let x=-5.
    Eliminate any answer choice in which x=-5 doesn't work.

    Answer choice A: |x|≤3
    |-5|≤3.
    5≤3.
    Doesn't work. Eliminate A.

    Answer choice B: |x|≤5
    |-5|≤5
    5≤5.
    This works. Hold onto B.

    Answer choice C: |x-2|≤3
    |-5-2|≤3
    7≤3.
    Doesn't work. Eliminate C.

    Answer choice D: |x-1|≤4
    |-5-1|≤4
    6≤4.
    Doesn't work. Eliminate D.

    Answer choice E: |x+1|≤4
    |-5+1|≤4
    4≤4.
    This works. Hold onto E.

    Only B and E remain.
    Answer choice B -- |x|≤5 -- includes every value from -5 to 5, but the needed range is only from -5 to 3.
    Eliminate B.

    The correct answer is E.

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