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Thread: How long is a piece of string?

  1. #1
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    Post How long is a piece of string?

    Hey guys,

    I am seriously stuck on how to answer this question.

    Any tips on how to answer these questions would be greatly appreciated!

    Please see the image below:

    How long is a piece of string?-string.jpg

    my attempt is that the length of string is 4x + 4y, but I feel like I am incorrect as it is not in terms of x. I'm just overall really stuck.
    Last edited by ifailedmaths; Mar 3rd 2019 at 10:53 PM. Reason: Include my working
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  2. #2
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    Re: How long is a piece of string?

    looking at the diagram it appears that

    $L(x) = 4x+4y$

    which is what you got.

    We're told the volume $V = 8000~cm^3$

    $V = x^2 y$

    $y = \dfrac{V}{x^2} = \dfrac{8000}{x^2}$

    $L(x) = 4x + \dfrac{32000}{x^2}$

    can you proceed from here?
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  3. #3
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    Re: How long is a piece of string?

    Hi there romsek,

    thankyou so much!

    is the dL/dx just asking me to differentiate the L(x)=4x+(32000/x2)
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  4. #4
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    Re: How long is a piece of string?

    Quote Originally Posted by ifailedmaths View Post
    Hi there romsek,

    thankyou so much!

    is the dL/dx just asking me to differentiate the L(x)=4x+(32000/x2)
    yep
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