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Math Help - Monotonic Functions and first derivative test

  1. #1
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    Monotonic Functions and first derivative test

    I am stuck with this topic which I just cant understand , I just know how to take derivative of an equation and also to find the critical points , the problem I am having is the increasing and decreasing part , I just cant understand how its done , i know that
    f'(x) > 0 (increasing)


    f'(x) < 0 (decreasing)


    but i just cant do it whenever i put some values I get wrong answers please help me out
    Last edited by mr fantastic; December 25th 2008 at 03:47 AM. Reason: Removed unsuitable language used to describe topic
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  2. #2
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    Quote Originally Posted by ahmedkamal29 View Post
    I am stuck with this topic which I just cant understand , I just know how to take derivative of an equation and also to find the critical points , the problem I am having is the increasing and decreasing part , I just cant understand how its done , i know that
    f'(x) > 0 (increasing)


    f'(x) < 0 (decreasing)


    but i just cant do it whenever i put some values I get wrong answers please help me out
    What exactly is your question? Lets say we want to find increasing intervals on [a,b]. We apply the first derviative test. We do as you said, we find f' and set it equal to zero (lets say that c is the zero). We then set up intervals based on the zeros (lets just say there is one for simplicity) so we should have something looking like (a,c)(b,c). Now we pick two numbers, a<d<c and c<e<b we then find f'(d),f'(e). If the value is positive that is an increasing interval and if it is negative it is decreasing. Does that make sense?
    Last edited by mr fantastic; December 25th 2008 at 03:46 AM. Reason: Modified quote to be consistent with editing
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  3. #3
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    Quote Originally Posted by ahmedkamal29 View Post
    I am stuck with this topic which I just cant understand , I just know how to take derivative of an equation and also to find the critical points , the problem I am having is the increasing and decreasing part , I just cant understand how its done , i know that
    f'(x) > 0 (increasing)


    f'(x) < 0 (decreasing)


    but i just cant do it whenever i put some values I get wrong answers please help me out
    Post the particular question that is causing you the trouble

    Re: "but i just cant do it whenever i put some values I get wrong answers".
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