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Math Help - Derivative help needed

  1. #1
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    Derivative help needed

    Hey guys I need help with this problem. I have to find the derivative using the formula:
    \lim_{h \to 0} \frac {f(x + h) - f(x)}h

    and the question is f(x) = \sqrt{x} - x

    please and thank you.
    Last edited by CenturionMonkey; February 11th 2008 at 07:52 PM.
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  2. #2
    is up to his old tricks again! Jhevon's Avatar
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    Quote Originally Posted by CenturionMonkey View Post
    Hey guys I need help with this problem. I have to find the derivative using the formula:
    (f(x+h) - f(x))/h

    and the question is f(x) = \sqrt{x} - x

    please and thank you.
    you mean you want \lim_{h \to 0} \frac {f(x + h) - f(x)}h

    so what is f(x + h) here?
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  3. #3
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    yes that's what I mean (I edited my post, I didn't know how to do the fancy math for that.) But I need the derivative of f(x)=\sqrt{x}-x
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  4. #4
    is up to his old tricks again! Jhevon's Avatar
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    Quote Originally Posted by CenturionMonkey View Post
    yes that's what I mean (I edited my post, I didn't know how to do the fancy math for that.) But I need the derivative of f(x)=\sqrt{x}-x
    yes, i know. that's why i asked you what f(x + h) was, you need to plug it in...

    you want f'(x) = \lim_{h \to 0} \frac {\sqrt{x + h} - x - h - \sqrt{x} + x}h

    \Rightarrow f'(x) = \lim_{h \to 0} \frac {\sqrt{x + h} - h - \sqrt{x}}h

    now multiply by \frac {\sqrt{x + h} + h + \sqrt{x}}{\sqrt{x + h} + h + \sqrt{x}} and continue
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