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November 14th, 2015, 05:25 AM  #1 
Newbie Joined: Oct 2015 From: blo Posts: 3 Thanks: 0  Critical numbers, concavity and inflection points
I really need help with this problem. I've been trying for hours, and I don't really understand this topic. If you could show me how to do this, and explain, then I would greatly appreciate it! Thank you! f(x) = (1x)(x+3)^2 A) What interval is it increasing and decreasing on? B) Find the xcoordinates of all local maxima of f, compute their average. C) Find the xcoordinates of all local minima of f, compute their average. D) Use interval notation to indicate where f(x) is concave up and concave down. E) Find all inflection points of f, compute their average. Last edited by skipjack; November 14th, 2015 at 07:12 AM. 
November 14th, 2015, 05:31 AM  #2 
Math Team Joined: Jan 2015 From: Alabama Posts: 3,264 Thanks: 902 
Where in the world did you get this problem? It is a fairly standard Calculus problem but you seem to be saying that you have not taken Calculus. Is that correct? Do you know what the derivative of a function is? Do you know what it means when the derivative is 0? When it does or does not change sign?

November 14th, 2015, 05:36 AM  #3 
Newbie Joined: Oct 2015 From: blo Posts: 3 Thanks: 0 
I'm in the process of taking the class now. This is the current topic that we are on, but my professor doesn't really speak English, so basically it's a "teach it to yourself class." I tried going to office hours with this, and he was no help then either. I can calculate the derivative of the problem, and I calculated out the critical points, but then I got stuck.

November 14th, 2015, 08:07 AM  #4 
Global Moderator Joined: Dec 2006 Posts: 21,019 Thanks: 2254 
As f$\,'\!$(x) is negative for x < 3, positive for 3 < x < 1/3, and negative for x > 1/3, f(x) has a local minimum at (3, 0) and a local maximum at (1/3, 256/27). As f$\,''\!$(x) = 6x  10, the point (5/3, 128/27) is the only point of inflection of f(x). Its coordinates may be found by averaging the coordinates of the local minimum and the local maximum. The function f(x) is concave up on ($\infty$, 128/27] and concave down on [128/27, $\infty$). 
November 14th, 2015, 08:20 PM  #5  
Banned Camp Joined: Jun 2014 From: Earth Posts: 945 Thanks: 191  Quote:
No, the function f(x) is concave up on ($ \infty$, 5/3) and concave down on (5/3, $\infty$).  
November 14th, 2015, 11:32 PM  #6 
Global Moderator Joined: Dec 2006 Posts: 21,019 Thanks: 2254 
Oops, thanks for the correction  I copied the function's value instead of the value of x. However, that value should be included by using square brackets: concave up on ($\infty$, 5/3] and concave down on [5/3, $\infty$). 
November 15th, 2015, 10:25 AM  #7  
Banned Camp Joined: Jun 2014 From: Earth Posts: 945 Thanks: 191  Quote:
No, that value should not be included. "A function $y = f(x)$ is concave up on an open interval if $y''$ is positive on the interval. And a function $y = f(x)$ is concave down on an open interval if $y''$ is negative on the interval." Source: http://www.millersville.edu/~bikenag...conc/conc.html They have to be open intervals. At x = 5/3, the value of the second derivative is zero. . Last edited by Math Message Board tutor; November 15th, 2015 at 10:32 AM.  
November 15th, 2015, 11:24 AM  #8 
Global Moderator Joined: Dec 2006 Posts: 21,019 Thanks: 2254 
Nice try, but here's another source that clearly supports my views. Math Tutor  Derivatives  Theory  Graphing My view is also supported on wikipedia, which gives a definition that doesn't require that the function is twice differentiable. I prefer this approach. Consider the function $y = x^{4/3}$. It certainly "looks" concave up everywhere, but is not twice differentiable at $x$ = 0. At least one other source, though, states "there seems to be no general agreement on a definition". 

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concavity, critical, inflection, number, numbers, points 
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