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Math Help - Having problems finding a value for X using derivatives.

  1. #1
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    Question Having problems finding a value for X using derivatives.

    Hi,
    I am really struggling with these types of questions. So if you've got the time, a way to solve these types of questions in general would be really helpful!


    Right, the specific question goes as follows:

    Given that P = 2x + 100/x
    Find the value of x for which P is minimum.
    Use the second derivative to justify your answer.
    Note: (x>0)



    I have no clue how to go about doing this, let alone what a second derivative is.



    Thank you!
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    Re: Having problems finding a value for X using derivatives.

    Quote Originally Posted by agehayoshina View Post
    Given that P = 2x + 100/x
    Find the value of x for which P is minimum.
    Use the second derivative to justify your answer.
    Note: (x>0)

    Can you differentiate 2x+100x^{-1} twice ?
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    Re: Having problems finding a value for X using derivatives.

    erm,
    2x + 100x^-1
    2x + 100x^-2 ?
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    Re: Having problems finding a value for X using derivatives.

    Quote Originally Posted by agehayoshina View Post
    erm,
    2x + 100x^-1
    2x + 100x^-2 ?

    N0! It is 2-100x^{-2}~\&~200x^{-3}.

    I think you need help that is far beyond what we can give.
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    Re: Having problems finding a value for X using derivatives.

    Quote Originally Posted by agehayoshina View Post
    I have no clue how to go about doing this, let alone what a second derivative is.
    If you do not know how to find a minimum or what a second derivative is, then you should not have been assigned this problem. You need a face-to-face talk with your instructor.
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    Re: Having problems finding a value for X using derivatives.

    oh.

    how come the 2x changes to just 2?
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    Re: Having problems finding a value for X using derivatives.

    Quote Originally Posted by agehayoshina View Post
    oh.

    how come the 2x changes to just 2?
    do you know what a derivative is? how to find it? what it tells you?
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    Re: Having problems finding a value for X using derivatives.

    As far as I understand it the derivative of 2x^2 would be 4x^1 so just 4x. Is that correct or am I totally wrong on this?

    thanks
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    Re: Having problems finding a value for X using derivatives.

    that's fine ... but why do you not understand that the derivative of 2x is 2 ?
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    Re: Having problems finding a value for X using derivatives.

    OH! i see. So would the derivatives of 2x+100x^-1 be

    2 - 100x^-2

    and

    2 + 200x^-3 ?
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    Re: Having problems finding a value for X using derivatives.

    not quite ... the derivative of 2 is 0
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    Re: Having problems finding a value for X using derivatives.

    okay. so just 200x^-3? ..also how can i use this to prove/find the minimum value of x for P?
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    Re: Having problems finding a value for X using derivatives.

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    Re: Having problems finding a value for X using derivatives.

    when u find a max and min point u derivate and set to zero then u put it on second derivate and se if its possitive i think its a min point and if its negative its max. wait for some1 to confirme im kinda tired so i can write wrong <.<
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    Re: Having problems finding a value for X using derivatives.

    Thanks @skeeter ! how would i apply this to the top question. I am mostly cleared up on the derivative area...but how do i find the minimum which they're asking for in the question?
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