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Math Help - max and min point

  1. #1
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    max and min point

    solve max and min range
    my progress derivate it and get 1/x+1-2x and find the roots x=1 and x=1/2 what is my next step?
    need a tips what i shall do next
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  2. #2
    fkf
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    Re: max and min point

    Quote Originally Posted by Petrus View Post
    solve max and min range
    my progress derivate it and get 1/x+1-2x and find the roots x=1 and x=1/2 what is my next step?
    need a tips what i shall do next
    We have
    f(x) = ln(x)+x-x^2 => f'(x) = 1/x+1-2x

    Searching for max/min by finding for which x we have f'(x) = 0. Hence
    1/x+1-2x = 0 => x = 1 and x = -1/2

    But we're looking in the range [1/4,2] and x = -1/2 is not in that range. x = 1 are in the range, therefore x = 1 is a valid solution to the problem. Now you can check if this is a max or min by using the second derivative or by studying if the first derivative is positive or negative around x = 1.
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    Re: max and min point

    Quote Originally Posted by fkf View Post
    We have
    f(x) = ln(x)+x-x^2 => f'(x) = 1/x+1-2x

    Searching for max/min by finding for which x we have f'(x) = 0. Hence
    1/x+1-2x = 0 => x = 1 and x = -1/2

    But we're looking in the range [1/4,2] and x = -1/2 is not in that range. x = 1 are in the range, therefore x = 1 is a valid solution to the problem. Now you can check if this is a max or min by using the second derivative or by studying if the first derivative is positive or negative around x = 1.
    i enter in the computer empty on min point and i get inorrect x=1 is max point and its correct but how do i solve min =S?
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  4. #4
    fkf
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    Re: max and min point

    Quote Originally Posted by Petrus View Post
    i enter in the computer empty on min point and i get inorrect x=1 is max point and its correct but how do i solve min =S?
    You have the value for f(1). But you also have the range [1/4,2]. Which means that the values of the function lies in this interval. Therefore you also have the values 1/4 and 2. And also the f(1).
    Last edited by fkf; November 7th 2012 at 11:39 AM.
    Thanks from Petrus
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    MHF Contributor MarkFL's Avatar
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    Re: max and min point

    Evaluate the function at the endpoints of the given interval and at the critical value within the interval. The largest of these is your absolute maximum and the smallest is your absolute minimum.
    Thanks from Petrus
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    Re: max and min point

    Ty mark! Tack fkf
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  7. #7
    fkf
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    Re: max and min point

    Quote Originally Posted by MarkFL2 View Post
    Evaluate the function at the endpoints of the given interval and at the critical value within the interval. The largest of these is your absolute maximum and the smallest is your absolute minimum.
    Since we have the range, we don't need to calculate the values for the endpoints, since the range is giving us the values for some x.
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  8. #8
    fkf
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    Re: max and min point

    Quote Originally Posted by Petrus View Post
    Ty mark! Tack fkf
    I did edit my post, but maybe you already got the point.
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  9. #9
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    Re: max and min point

    Ty you prob also read math in Stockholm university?
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  10. #10
    fkf
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    Re: max and min point

    Quote Originally Posted by Petrus View Post
    Ty you prob also read math in Stockholm university?
    Nope, been studying geology there (distance course). I just started to study civil engineering in applied physics (or maybe it's called engineering physics).
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  11. #11
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    Re: max and min point

    Quote Originally Posted by fkf View Post
    Since we have the range, we don't need to calculate the values for the endpoints, since the range is giving us the values for some x.
    I interpreted the information given that we have \frac{1}{4}\le x\le2. I assumed Petrus meant domain instead of range.
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