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sqrt(80) +

This topic has 2 expert replies and 0 member replies
boomgoesthegmat Senior | Next Rank: 100 Posts Default Avatar
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sqrt(80) +

Post Wed May 04, 2016 4:29 am
Elapsed Time: 00:00
  • Lap #[LAPCOUNT] ([LAPTIME])
    sqrt(80) + sqrt(125) =

    A) 9sqrt(5)
    B) 20sqrt(5)
    C) 41sqrt(5)
    D) sqrt(205)
    E) 100

    OA: A

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    Post Wed May 04, 2016 5:46 am
    boomgoesthegmat wrote:
    √80 + √125 =

    A) 9sqrt(5)
    B) 20sqrt(5)
    C) 41sqrt(5)
    D) sqrt(205)
    E) 100

    OA: A
    IMPORTANT RULE: √(xy) = (√x)(√y)

    So, √80 + √125 = √(16 x 5) + √(25 x 5)
    = (√16)(√5) + (√25)(√5)
    = 4√5 + 5√5
    = 9√5
    = A

    Related Resource
    The following video covers the concepts/strategies that are useful for answering this question:
    - Simplifying roots: https://www.gmatprepnow.com/module/gmat-powers-and-roots/video/1038


    Cheers,
    Brent

    _________________
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    Post Wed May 04, 2016 5:51 am
    boomgoesthegmat wrote:
    √80 + √125 =

    A) 9sqrt(5)
    B) 20sqrt(5)
    C) 41sqrt(5)
    D) sqrt(205)
    E) 100

    OA: A
    Another approach is to ESTIMATE

    We know that √81 = 9, so √80 is a little bit less than 9
    We know that √121 = 11, so √125 is a little bit more than 11

    So, √80 + √125 ≈ 9 + 11 ≈ 20

    When we check the answer choices, we must keep in mind that √5 ≈ 2.2

    A) 9√5 ≈ (9)(2.2) ≈ 20 NICE!!
    B) 20√5 ≈ (20)(2.2) ≈ 44 NO GOOD. ELIMINATE
    C) 41√5 ≈ (20)(2.2) ≈ TOO BIG. ELIMINATE
    D) √205. We know that √225 = 15. So, √205 is too small. ELIMINATE
    E) 100 ELIMINATE

    Answer: B

    Cheers,
    Brent

    _________________
    Brent Hanneson – Founder of GMATPrepNow.com
    Use our video course along with Beat The GMAT's free 60-Day Study Guide

    Check out the online reviews of our course
    Come see all of our free resources

    GMAT Prep Now's comprehensive video course can be used in conjunction with Beat The GMAT’s FREE 60-Day Study Guide and reach your target score in 2 months!

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