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Homework Help: Concavity, inflection ps, intervals of F.

  1. Jul 12, 2010 #1
    1. The problem statement, all variables and given/known data

    (a) Find the intervals on which is increasing or decreasing.
    (b) Find the local maximum and minimum values of .
    (c) Find the intervals of concavity and the inflection points.


    3. The attempt at a solution

    a) f ' =6x/(x2+3)2

    6x=0 => x=0

    What concluusion can I draw from this data about increase/decrease?
    I am asking because my function actually does not go below y=0, so I thought that it does not decrease at all. My answer is that f is only increasing on intervals for which f'(x) > 0.

    b) local minimum is at (0,0). no local maximum.
    c) inflection points: x=+-1.

    Stuck with concavity!!
  2. jcsd
  3. Jul 12, 2010 #2
    So setting the first derivative equal to 0 gave you a local max or min; finding the sign of f'' at x = 0 will tell you which one it is.

    Now you need to find f'' and set it equal to 0 to find the concavity next. The points of inflection are where the concavities change; with two points of inflection, x = -1 and x = 1, you have three intervals on which to check the sign of f'' to determine the concavity of f on each of those intervals.
  4. Jul 12, 2010 #3


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    Last edited: Jul 12, 2010
  5. Jul 12, 2010 #4
    I am sorry about that. I forgot about that post, thank you for reminding.
  6. Jul 12, 2010 #5


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    Ok, so given Bohrok's hint, what are the intervals of concavity? Where is f'' positive and negative?
  7. Jul 12, 2010 #6
    My problem is that I cannot tell the difference between the domain, concavity, increase and decrease. There are different formulas, different approaches. I have been searching for two days, contacting my friends, looking through math forums.

    If someone can just show me how to do this one problem, I will do the next ten on my own in my homework, please.
  8. Jul 12, 2010 #7
    You don't need to consider the domain of any functions to find concavity.

    The three intervals I was talking about are (-∞, -1), (-1, 1), (1, ∞). Each one has a concavity, and to find them, you look at whether f'' is positive or negative in each interval.
    For (-1, 1), f'' is greater than 0, which means that f is concave up on that interval. If f'' is less than 0, f is concave down on that interval.
    Can you finish the rest?
  9. Jul 12, 2010 #8
    That is awesome. Finally, after so many sleepless days and nights, I completed one of my problems. Thank you a lot for your wonderful support, people of PF.
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