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Thread: Help Finding Perimeter of Rectangle

  1. #1
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    Help Finding Perimeter of Rectangle

    Hello,
    I am studying to take a math placement exam in two days. There's this one geometry problem that I cannot figure out how to solve. I watched a video of someone solving it but I don't understand what they're doing after a certain point. Here's the problem:
    Help Finding Perimeter of Rectangle-screen-shot-2017-04-17-3.26.18-pm.jpg

    I don't know if it's just me but I think his approach to solve it is way too complicated. Does anybody know another way to solve this problem other than what he's doing? I know how to factor well but even with that what he's doing is still confusing me. I'll leave screenshots of what the guy in the video did but I am drawing a blank in this one.
    (I understand what he did up to this picture, after that I don't really know why he's doing all that)
    Help Finding Perimeter of Rectangle-screen-shot-2017-04-17-3.26.39-pm.jpg


    Here's the rest of his work:
    Help Finding Perimeter of Rectangle-screen-shot-2017-04-17-3.29.03-pm.jpg
    Help Finding Perimeter of Rectangle-screen-shot-2017-04-17-3.29.17-pm.jpg
    Attached Thumbnails Attached Thumbnails Help Finding Perimeter of Rectangle-screen-shot-2017-04-17-3.28.19-pm.png  
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  2. #2
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    Re: Help Finding Perimeter of Rectangle

    $l \times w = 96$

    $w = l - 4$

    $l(l-4)=96$

    $l^2 - 4l - 96 = 0$

    $l^2 - 4l + 4-4-96 = 0$

    $(l-2)^2 -100 = 0$

    $(l-2)^2 = 100$

    $l - 2 = 10$ (we can ignore the negative solution)

    $l = 12$

    $w = l-4 = 8$

    $P= 2(l+w) = 2(12+8) = 40$
    Thanks from whytbh
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